My clan citadel is tier 7. All plots are tier 7. Up until 2-3 weeks ago no ones been able to claim cooking bxp (see picture below). I'm definitely 40+ and the plot definitely isn't dilapidated.
It seems a few people have been having similar issues with claiming bonus xp, although not specifically with the cooking plot. I would suggest that you submit a bug report - do it while you are in the citadel, and make sure to state the name of your clan in the report