Aptx via Bluetooth does not work properly

Hello Community,
I have been using e OS on my oneplus 5 for a few days, the latest R release.

I noticed that the Bluetooth transmission with aptx does not work properly. The sound then sounds very muffled and too bass-heavy. If I turn off aptx in the Bluetooth settings, the sound is normal.
I only have one Bluetooth receiver that can handle aptx (Avantree RC500), so I can’t say exactly how it looks with other devices.

I have also reset the Avantree, but no improvement.

I had this over a year ago with my previous custom Rom (crdroid), but that was quickly fixed with an update.

Maybe someone can test this with an aptx-enabled device?

Translated with www.DeepL.com/Translator (free version)

Did you try Settings > System > Advanced > Reset options > Reset Wi-Fi, mobile & Bluetooth?

Unfortunately, it doesn’t help.
I have already reset the Bluetooth receivers.
And with the other custom rom, aptx still works perfectly (I have two Oneplus 5).

I am not good on Bluetooth. It would be good to know where exactly to look for a comparison beteeen the two situations.

Is the other LineageOS or Lineage derived, same Android version, perhaps give both build numbers in case it helps trigger where to look?

It is crdroid and it is also based on LineageOS and it is also Android 11.
The build date is one month younger. But, as I said, the aptx problem has not occurred since ~March.

Unfortunately, nothing has changed with 0.21 R. Aptx still sounds awful.
I just wonder what crdroid does differently…