Weird Quirk With HEICs in Lightroom

[robertoleonardo]

So I've been using xmp sidecars to get my metadata into lightroom with HEIC files since it still doesn't support direct reading of the metadata.  But i just noticed something weird and wondering if anyone has any ideas -

i did a big lightroom reorganization and uploaded everything to lightroom CC, and now it's time to sync it all back down to lightroom classic for local library management, which means the heics get sorted into folders without sidecars and thus, initially, my heics don't have keywords or anything in lightroom classic.  My process has in the past to create an xmp once they're sorted and then i can read the 'heic' (xmp) metadata from within lightroom and it brings all the info back into my LR library.  Of the 800 or so HEICs that have synced down so far - 750 of them are behaving as expected - no keywords or anything, because i haven't yet created the sidecars.

BUT then there are 30-40 HEICs--with no sidecars--that somehow are already showing keywords in lightroom.  I'm looking at the metadata and can't see any relevant difference between those that work without a sidecar and those which don't - the keywords are written to xmp:subject in both types. 

Anyone have any idea why some of my HEICs are working without an xmp?  If it's a trick that i can replicate - i'd love to do away with sidecars...

(relevant - when i started syncing - i did NOT create a new library but i DID clear out all photos in it prior to beginning the sync.  So i suppose it's possible the keyword data just somehow stayed in the library  -but it's odd that it's only doing it on a few photos)