It looks like Mixxx is unable to read BPM / tempo tags from MP4 files. I've tested this with both the current stable 2.0.0 download and latest built from master. It reads BPM from mp3 files but for all m4a files, BPM is left blank.
Looking into this, this appears to be because of the atom data type check enabled by the ifdef at https://github.com/mixxxdj/mixxx/blob/master/src/track/trackmetadatataglib.cpp#L1587 . The problem is that taglib doesn't set the atom data type for the 'tmpo' atom, only for freeform atoms (https://github.com/taglib/taglib/blob/4801fbb927c15afab864f7b2f02ad2e07c332ecc/taglib/mp4/mp4tag.cpp#L277) so the type of the 'tmpo' atom is always undefined, meaning the BPM is never read. Removing the check (ie. removing the #define) causes tempo values to be read correctly.
The tmpo atom is always parsed as an int in taglib (https://github.com/taglib/taglib/blob/4801fbb927c15afab864f7b2f02ad2e07c332ecc/taglib/mp4/mp4tag.cpp#L77) so I think the best solution here is to say that atomDataType is only valid for freeform atoms and remove this check (Mixxx doesn't do any equivalent checks for other atom like track numbers).
I'd obviously be happy to send a pull request for this, although it's a trivial patch.
Thanks, I'll revisit this code before releasing the beta! Not sure why I introduced the type check reading my comments ;)