I’m still catching up in here but the information as promised:
From reset today, dead ends in Underspire will not be longer than 6 tiles.
This is the way it was always intended to work - there was a bug which meant that a dead end could try to fill the entire map.
There are changes coming to Underspire in a future update which will see this number changed again - the details will be in the patch notes for the update those changes are in.