I have been using Mixxx for some time but I still have the same problem despite version updates.
The beatgrid shifts all the time. It is well adjusted at the start (automatically or manually) but towards the end or sometimes the middle, it is completely offset.
I have this problem with the latest stable and beta version. Even when setting the BPM manually and locking it, the problem persists. On the other hand, I do not have it on all the musics. The mp3s used are not the cause because I do not have this problem on other software (VirtualDj, Transition DJ, DJuiced …).
I am using Mixxx on Ubuntu 20.04 LTS.
Do you have a solution ?