Departed RTM via LCY/(SNN)/JFK (BA1) to MIA last week. Staff at RTM did not know about the preclearance facilities in SNN and manually short-checked my bags to JFK because 'I had to clear customs there'. I asked if she could check my bags all the way to MIA and explained about the fuel/preclearance stop in SNN. I was told she had 'clear instructions' to short-check all bags to the first point of entry in the US due to customs regulations.

I ended up picking up my bags at JFK T7, dragging all of my stuff in the skytrain to T8 and recheck it at the AA check-in.

Wrote a quick (constructive) message to BA and suggested to provide some additional instructions to RTM ground staff regarding BA1 (I can imagine there is not a lot of RTM-US traffic). Received a friendly reply that RTM ground staff will be briefed about the customs/immigration in SNN and 10K Avios were added to my account. Will check in a couple of weeks if my bag indeed will be checked to my final US destination from RTM!