Dig Map terrain generation - More than 50 monsters remain - unreachable

Ran a Dig Map(T13) just now, it was extremely small and had very few (reachable)mobs. After killing the boss I went back thinking I missed a passage to the rest of the map somewhere but there was no such thing.

Circled around the map like 10+ times but couldn't find any way to progress. /remaining told me more than 50 remained, also shown in the top right of the screenshot(includes a view of the map and the bug report reference number):.

Also might be worthy of note that I did not encounter any Syndicate activity.