Editing Markdown fields don't work

We've had several reports that editing Markdown (issue notes, MR descriptions) were not working. Clicking save succeeds but the rendered Markdown doesn't show the new changes. When clicking "Edit" again, the new edit is actually there in the Markdown.

Upon investigation, this was due to the markdown field being updated (note, description), but the cached HTML column was not regenerated.

This started happening after the deploy (I think 12.9.202002261506-cf91edbb.76b8c645816). But we didn't do anything related there except bump the markdown cache version which would've forced regeneration of these HTML columns when they are viewed.

Editing the object again and adding extra characters to trigger the regeneration of the cache works. So this has been very difficult to reproduce.