I'm hoping it's something easy to fix and not a problem embedded in the (NTT Docomo) ROM, but when I share my data connection as a hotspot, my data connection drops as soon as another device makes a connection with my phone.

Turning hotspot sharing off restores the data connection immediately. Sharing worked in the past, so I am unsure what might have happened to create this issue.

I've tried sharing with different SIMs and networks - same problem.