Not particularly scientific but I'll often just End Task on Prep (and close other instances of Tableau Desktop, etc.). Sometimes doing a full restart of the software and making sure that no other Tableau software is open can make a difference. In effect, turn it off and on again and hope for the best.
Yes I have been doing the less scientific approach multiple times and then restart with the same end result. I have reverted back to 2018.1.1 and have no issues.
Sounds like it's maybe a bug then. The data does seem to hang a lot but they are supposed to be constantly updating the software so hopefully it will be resolved soon.