When I ported a world from 18w50a to 19w02a with existing player data on it, any tools on that playerdata file would break blocks super quickly and their recovery time/attack speed would also be extremely quick. Even new tools given using /give would do the same thing. The armor bar would also not appear anywhere on the GUI. I do not know what caused this bug, but I hope you will find it, and you will squash it.
I just discovered changing "DataVersion:1919", in a playerdata NBT file, to "DataVersion:1921" works but it's inconsistent.
Please fix this soon
This bug isn't present in 19w07a, so idk who forgot to mark it as solved.