SUMMARY

Had been going to do a bug report about layer thumbnails empty [shows a single black pixel square with white outline] in most recent 5.3 appimage ran (krita-5.3.0-prealpha-de10297103-x86_64.appimage) ... tried to run a later 5.3, to note what to include in bug report, but unfortunately segfaults (have done bug report ... https://bugs.kde.org/show_bug.cgi?id=485252). 

On trying versions had missed while taking a break, e.g. 5.2.0/5.2.1 betas and finals, and 5.2.2, found same issue of layer thumbnails empty. 

Using krita-5.2.0-prealpha-e41db7b620-x86_64.appimage from before, and it works fine, including layer thumbnails. 
Have done various searches, including on krita forum, but can't seem to see anyone with same issue, or any bug reports on it. Not sure what changed from 5.2.0-prealpha onwards, but can't run anything since.  

STEPS TO REPRODUCE
1. open variety of appimages
2. no layer contents show ... single dark pixel in centre of each layer thumbnail

OBSERVED RESULT
empty layer thumbnails

EXPECTED RESULT
contents of canvas show in layer thumbnails

SOFTWARE/OS VERSIONS
MX Linux 21 xfce, running Mate 1.24.1, kernel 5.10, x11, mesa 20.3.5
amd ryzen7, radeon rx5700xt, 32 ram
Qt Version: 5