Hello!
If you guys have the same problem with me, lin's friendship stuck at level 16. Upgrade DUNGEON, after you finish upgrade it, go to dungeon. You will meet lin in there and trigger event to increase her friendship.
If you don't have the dungeon, this event will never triggered. So you will always stuck at level 16. Idk if it can counted as bug because there is no clue, if you don't have the dungeon.
For my feedback i hope the developer put a hint/clue to upgrade dungeon (if the player doesn't have it yet) when lin's level reach 16.
I hope it would help.