This is what I did to try to break rhythmbox again:

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while rhythmbox is open, in Nautilus I made a temporary music library folder with just one song (as opposed to 30,000 songs, which seems to make the debug output files massively long)

changed the library folder in rhythmbox settings and closed rhythmbox

renamed the ~/.local/shared/rhythmbox/rhythmboxdb.xml file to a backup copy, to start from scratch

opened rhythmbox (output file 01 started)

in nautilus, renamed the song file (the only song in the library); rhythmbox seems to update the file properties correctly

in nautilus created album folder and dragged file in the new subfolder; in rhythmbox, one file still shows, but in the properties dialog, the location is now /home/dg/Desktop/(null)

opened the file in cowbell, changed tag info, saved the changes (which also renames the file itself); still no "missing file" status showing in the rhythmbox sidebar, still only one file showing up, location still /home/dg/Desktop/(null)

opened file with soundconverter, to convert from 192 bitrate to 128 bitrate; this creates a new file in a subfolder; no updating in rhythmbox yet, so went to menu bar to open preferences (which historically has appeared to force a rescan of the library); that did nothing, so i unticked and attempted to retick "watch library for new files" and rhythmbox crashed (in the terminal, the message was "segmentation fault")

restarting rhythbox (output file 02 started)

same erroneous file (just one though) shows in library window, try to play it, but it will not play

in the settings, the "watch library for new files" box did not get ticked before the segfault, so reticking it now; library does not update in rhythmbox

closing rhythmbox via window title bar button; does not stop rhythmbox though (does not terminate in the terminal)

open system monitor and rhythmbox is listed as sleeping with channel being futex_wait_queue_me; selecting end process

restarting rhythmbox  (output file 03 started)

now the library shows 3 files, 2 of which exist

in nautilus, deleting the original 192 bitrate file, dragging the new 128 bitrate file up from the subfolder, and deleting the subfolder; the rhythmbox library does drop one file, but still shows the nonexistent file at /home/dg/Desktop/(null); the rhythmbox side pane does update to show a missing file, which i right click and remove

in nautilus I rename the file; rhythmbox updates it correctly

in nautilus, I make a new folder and drag the file into it, and delete the old folder; the one correct file now shows up in the rhythmbox library at /home/dg/Desktop/(null)

restarting rhythbox (output file 04 started)

I try playing  the "null" files and then the GUI seems to go into an endless loop; i click the window title bar button to close rhythmbox, and then a third file shows up and the loop stops

i try to play the newly displayed file and the rhythmbox window dims and seems frozen

I clicked the x in the window title bar and selected "force quit"

restarted rhythmbox (output file 05 begun)

the correct file does now play, the two "null" files are still listed, and no "missing files" are listed

closing rhythmbox, done trying to break it

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Unfortunately, I cannot figure out how to replicate the situation where the old 192 bitrate file still shows up (not at a "null" location, but at the actual pa that did in fact exist a few minutes ago)

Hope this helps even without that additional condition being replicated