Regarding my XIAO_nRF54L15, it’s highly likely there’s a hardware issue.
I’m observing signal levels by placing a digital scope probe on the SWDIO and SWCLK pads on the back. The SWDIO pin on the XIAO that cannot upload remains constantly LOW with no changes. The SWDIO pin on the XIAO that can upload is normally HIGH and changes between HIGH and LOW according to the data.
Referring to the circuit diagram, I suspect there may be an unstable connection.
U4 (ATSAMD11) PA02 <----> U12 (UM3301) B
U12 (UM3301) A <----> U6A (nRF54L15) SWDIO
<----> SWDIO pad.