The outage caused by the upgrade of cgroup-bin upgrade (see #53800) took a while to figure out and fix.
Running the "convert" job by hand, with the arguments logged in thumbnail.log didn't produce any errors. Running what MediaWiki runs, limit.sh, was immediately revealing but figuring out the right arguments to it took some effort.
I propose that MediaWiki should capture the stdout/err of the convert jobs and log it (possibly the last 2-3 lines). I'm imagining this would also be useful for correlating e.g. imagemagick errors.