FYI, I’m seeing the same not working/heating issue with 2 of my expansion boards when attached via usb connector.
Seems the 3.3v line is being pulled down to .82v when the Xiao BLE is attached to the expansion board. It’s 3.2v when not. It was working just fine, then stopped working. It could be the pogo pins not making good/intermittent connection while power is applied is toasting the power regulator or some such.
A related post about two 3.3v regulators tied together. With my next expansion board, guess I’ll try what is suggested in the post and remove the Xiao, 3.3v pin from mating with the expansion board.
Per this post I cut a trace on the expansion board, I can now use the expansion board with JLink SWD (via Pogo pins). I did have to connect the Jlink 3.3v input directly to the Xiao 3.3 rather than along side the other SWD connections. I expect the other board functionality is lost though.