Level -1 troops in delve

Platform, device version and operating system:

Screenshot or image:

What you were expecting to happen, and what actually happened:
Started a delve at level 320. Troops had a ridiculously low amount of hp. After clicking on one they showed as level -1. After I beat them I got a cliff error and lost my delve attempt and lost all rewards.

How often does this happen? When did it begin happening?
Just once so far

Steps to make it happen again
No clue. This was my second delve attempt for today in the same faction. Third attempt was fine.