One database in AlwaysOn Group not truncating log file
I would like to comment but couldn't due to the lack of reputation. 

Is your secondary node being enabled read-only? And if so, did you check if there's any blockings on the secondary node? 

Another thing that you could check is if there's any maintenance jobs running.

And... could you try to make everything asynchronous mode to see if it helps? Once log reduced, you may set it back to synchronous mode. 


  ... I've created a ticket to ask for this particular database to be removed from it and then added again. However I'm not sure if this;
  a) will fix the problem 


Last but not least if everything doesn't work, yes, you can remove that particular DB from the AG group. It should fix the issue. Until the log has reduced, you may add it back to the AG group. Removing DB from the AG group shouldn't cause to any impact as applications should be connecting using the Listener. 


  b) a temp fix (truncate once, then slowly start growing again)


Yes, it might occur again if we don't figure out the root cause to it. And you probably would have to repeat the whole procedure in removing/adding back to AG group, though it shouldn't cause to any production impact.