[2022] Missing Pre-engineered Heatsinks

I did some grinding to get the mats required to unlock and buy 4 pre-engineered heatsinks from the material trader in the Leesti system.

After each unlock, I went to Outfitting and added the new heatsink to my Diamondback explorer. The plan was to then swap them out (NOT sell them) for the original spec of utilities I had, go do some other stuff and then transport the heatsinks to a couple of other ships I had in other spaceports.

When I swapped them out, 2 have just disappeared! They don't show in my stored modules at all.

Anyone else had this problem?