Ok, bug is about 3 year old, so I guess there is not a lot interest in fixing it (too bad, given it is easily reproductible).
On the bug page, someone mentions disabling MMAL acceleration avoids the issue. How do you disable this acceleration (and is it even feasible on RPi 4)?
MMAL has gone. The bug appears to be a buffer overwrite (e.g. buffer allocated is smaller than amount written).
But they tend to have fairly random symptoms - often changing something unrelated (like disabling MMAL) may make the issue appear or disappear.