When I placed my vessels with ore down in my pit and save & exited before lighting the fire, after loading back into the world the items vanished, and /data tells me the block only has block id "tfc:placed_item" with no data of what items they were.

Just checked, none of my items I placed onto the ground with V saved.

Ah, I think this is due to the switch from save to saveAdditional in 1.18, I forgot to check that before the recent build.