the losing of collection happens 2 me all the time now when i move any sim into an existing household, the whole house resets with sims on front sidewalk and collections blank, i have to manually put all fish from tanks, items on shelves back into any sim's inventory, unfortunately, if a sim learned a fish like treefish thru conversation it no longer appears so they lose out
when did this problem start... back in oct 2019 when ea customer service told me to reinstall all packs when the seasons calendar was having that disappearing holiday glitch, which the reinstall didnt fix the glitch,they later released a patch in nov fixing the glitch. since 2014 never had problem merging,then reinstalled now have to live with it.
But back to your lost collection, if you did reinstall recently and moved a new sim into an existing household and it happened, then you probably have the same problem as me and have to put the items back in any sims inventory