I have done this before several ways which is why it is very frustrating now.
I read all of Add transparency to an existing PNG but didn't try the "Curves on the Alpha Channel" suggestion from Demis. It's usually enough to (1) Add Alpha Layer and (2) Color to Alpha (2) Export as png or jpg.
I have other images like this one that upload correctly: https://oldcrowhempco.com/shop/green-lotus-hemp-heat-rub/
But then this one I put through the same process, it looks it has trans. backgrounds, but then when uploaded, it's displayed with white background: https://oldcrowhempco.com/shop/green-lotus-premium-hemp-oil-salve-250-mg/
One thing I noticed that I thought was odd when I downloaded the one that's appearing correctly and imported back into GIMP, Mode is Indexed not RGB. However, while I have not been able to capitalize on this knowledge yet, it seems relevant.