What you were expecting to happen, and what actually happened:
Achievement should unlock since I have 500+ troops. Instead it’s listed as locked w/ completion percentage show as 100%.
How often does this happen? When did it begin happening?
Always. Started happening when 5.1 update was introduced.
Steps to make it happen again
Presumably have 500+ troops then update to 5.1. I don’t know of any way to downgrade on the platform & try to reproduce.