Favories disappear from sidebar every so often

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac, reboot normally and test to see if the problem persists.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Loudlaf wrote:

My favorites every so often disappear from the sidebar without warning. (MacBook Pro 2019, Ventura 13.1.3) Does Apple know about this? Is there a fix?

If you hover the cursor next to the word Favorites you'll see a downward pointing caret. If you accidentally click on it the Favorites list will hide.

Are the bookmarks only on iCloud and not on your Mac? I ask because iCloud was having problems last week is why I ask!