Oops, it was the obvious (power). I was convinced it wasn't power because it was previously working but looks like my PC USB port was having issues.
Thanks.

Device was previously working but now stuck in a boot loop.
Tried reflashing both default buildroot and Ubuntu images with no change.

Full uart output here:
https://pastebin.com/wAfyFJKG

Seems to reset at:

[ 3.470071] libaes: exports duplicate symbol aes_decrypt (owned by kernel)
insmod: can't ...