Having a decor 26 plant in a room doesn't seem to satisfy the requirement of a Great Hall for "Decor item: 20+ Decor". Save attached (there's one obvious meal hall, it's below and to the left of the start.) I've only tested Wall Pots currently, but I believe flower pots had the same problem in previous versions.
Steps to Reproduce
Build a great hall using a wall pot to try to satisfy the decor requirement.
This issue has been fixed in either a future version or updated in a previous version.