EDIT: I misunderstood the Pop Shop GUI. I actually DID install using flatpacks.
Since my post, I found user monteverdi had the same problem. Timeline only renders when resizing window - #19 by chrisisbd
I removed ~/.audacity-data but it did not help.
This fixed it for me
GTK_IM_MODULE=audacity
Is it a real fix? What caused the problem to suddenly occur?