"Search all fields" does not search within comments
Rhythmbox 3.4.3 via Debian 10 (Buster) here.
The main interface offers a search bar which is labeled "Search all fields".
Since I enabled comment column in the lower half, I assume that "all fields" refer to either all visible fields or all fields that are not even visible. Either way, I assume to be able to locate words and phrases within comments.
Actual result: nothing is found which is stored within comments.