This is a trouble which was introduced by Xiaomi in a security update for the Xiaomi Mi 9, whose review has already been done here on Skooter Blog. After this update, or my 9 can no longer connect to the sound system of some Honda cars (Honda HFT) getting in an infinite connection and disconnection loop.
Disabling the make/answer function with the car sound system alleviates the problem by allowing connection after a few seconds or even minutes, but in this case it is only possible to use sound to listen to media.
I don't remember exactly which ROM version the problem started on, but it was in some of the revisions of V12.0. My Me 9 still received at least one more update after the one that caused the problem without it being solved.
After that unfortunately the updates stopped. I ended up finding out that officially Xiaomi was already on V18.104.22.168, but this one didn't show up for me. I even tried to force it in various ways via OTG without success. I even tried downloading the update to just install by the updater and I was unsuccessful too.
In this endeavor I also discovered that, despite my mi 9 have been sold as containing a global ROM, in fact he was with a European ROM (EEA), as noted in part of the installed ROM string (“RFAEUXM” em vez of “RFAMIXM” da ROM Global). But not even the newest version of official European ROM worked.
In an attempt to solve the Bluetooth problem, I decided to install Xiaomi's latest official global ROM, a V22.214.171.124.RFAMIXM, even if it had to be done via bootloader or TWRP. For that I had to unlock the bootloader. I used XiaoMiTool V2 for this. it wasn't smooth. There were several attempts, I had to log into the Xiaomi account and ask for an unlock code, but in the end it worked. Moreover, know that when installing a new ROM, Xiaomi locks the phone until logging into the account and releasing. It's an interesting security measure, but i hope this server never goes down.
Unfortunately V126.96.36.199 does not solve the Bluetooth problem, and in fact it even seemed a little unstable to me on my cell phone, but maybe I didn't clean everything I should have before using it. I doubt that Xiaomi will still release a new ROM for the Mi 9. The device is already reaching your 3 years since launch, so it's hard to expect much from the manufacturer, whatever it is. I just find it regrettable that the last ROM is one that keeps a bug that has already been detected and reported in several previous versions. It was certainly Xiaomi's unwillingness to solve the problem.
Since it was in the rain, I decided to get wet once and for all and started testing totally alternative ROMs. I installed them in different ways: sometimes copying the file and then installing directly from TWRP, sometimes using sideloader, sometimes using their own XiaoMiTool V2, which also uses sideloader. sometimes it worked, others the cell phone was in boot loop, I couldn't find a pattern.
What I realized with ASOP Alternate ROMs, many based on Pixel, is that they solve the problem of Bluetooth, but they don't support Xiaomi hardware very well. They are cheap standard cameras that only use one lens., call google assistant button not working, inside other problems. The camera can even get around with the GCam, but it's a hell of a hassle to find the ideal version and even then it's not as good as it should. Even though I hated MIUI at first and immediately installed the Nova Launcher, I have to admit that she has some virtues.
Of all alternate ROMs, the only one I used a little more was the ArrowOS (Arrow-v12.0-cepheus-UNOFFICIAL-20211028-GAPPS.zip). I even stayed with her for a day., install all apps, but I finally gave up because of the limitations and differences of MIUI.
I realized that I wouldn't be satisfied with a ROM that didn't fully support the hardware, so I went for the ROMs that are based on MIUI. The first one I tried was MiuiMiX (MMX_2.2.0_cepheus_V188.8.131.52.RFACNXM_v12-11_23000_9_20211201.zip). I even liked her, but unfortunately it didn't solve the Bluetooth problem either.. Furthermore, it comes with root, what causes some banking apps to not work. The options of Magical to hide root works for some apps, but not for everyone: C6Bank, Abastece Aí and PicPay are some of the problematic ones that do not accept root.
A solution ended up coming up with the ROM V12.5.6 from xiaomi.eu (xiaomi.eu_multi_mi9_v184.108.40.206.rfacnxm_v12-11.zip), also based on MIUI. Moreover, it is based on MiuiMiX itself, so i already installed it without putting faith that it would solve the bluetooth problem. But to my surprise it solved the problem and works perfectly with camera and all Mi features 9.
The only problem I remember with V12.5.6 is that Nova Launcher started having problems pressing the button to see the open apps and trying to reopen some of them. Many did not reopen. I ended up removing Nova Launcher and leaving only MIUI anyway. She has improved a lot with the version 11 and now there's one drawer, something the version 10 didn't and it was the main reason I installed Nova Launcher in the first place.
So here's the solution for those who have a Xiaomi Mi 9 and a Honda and doesn't want to have to change his cell phone, let alone a car: the stable version V12.5.6 from xiaomi.eu:
To be honest, I don't even know if the problem was fixed by Xiaomi itself in its V12.5.6, which was not and probably will not be officially released for Mi 9, or if it was something that the xiaomi.eu community itself introduced. I don't even understand why the problem was not solved with MiuiMiX, since it is based on xiaomi.eu, but the important thing is that the problem went away.