I did try the “Delete Jmix…caches” option, to no avail. And no, they’re not the same version - one is 1.5.3 (the nonworking project) and the other is still on 1.5.2.
idea.log had no interesting entries.
Unfortunately, in an attempt to downgrade to IDEA 2023.1 to see if that would fix the issue, I somehow made everything work correctly. I first copied “existing IDEA directory” to “existing IDEA directory backup” in case this was a horrible idea. Then I extracted the 2023.1 archive over the “existing IDEA directory” - which produced all manner of issues. So I deleted “existing IDEA directory” and then renamed “existing IDEA directory backup” (which was merely a cp -R copy of "existing IDEA directory) back to “existing IDEA directory” … and then everything is working.
I don’t understand this behavior at all, but everything is working (including the first issue with auto-suggest), so I don’t know. I wish I knew WHY that made things work.