At about 42 dwellers, I got around to building a radio station for Three Dog to work in. I'd say he's pretty happy.

I'm not entirely convinced that this is a bug so much as it is an unintended (and irksome) effect of the radio station granting a +1% point to all vault dwellers.

Before having placed Three Dog (Or any dweller) into the radio station, the vast majority of my vault was at 100% happiness and in fact I believe my lowest happiness percentage in a dweller was 94%.

I wasn't so much interested in the vault wide +1% point since I had a happy vault, but I did notice that the next day when I achieved in getting my final dweller to 100%, my overall vault happiness was stuck at 99%. I immediately guessed that there was something funny going on with the radio station's +1% not playing nice with my overseer grading system, and after removing my dweller in the station, all 42 dwellers revealed a rating of 99% happiness.

I'm wagering that it's the radio station's +1% point behaving like a blanket. It shows 100% for an individual dweller, but as far as the accumulative vault happiness is concerned, it has no effect. I expect I could still achieve an overall happiness of 100 if I succeeded in rushing rooms for that final 1% for every dweller, but with 42, failing a rush would be quite an inconvenience.

TL;DR

I suspect that removing a dweller from a radio station will cause dwellers to fall to 99% happiness even if they were at 100% before the radio station was being manned. This seems unintentional, as it makes achieving and holding onto the overseer grade of 100% very difficult.

I have no clue if this has been previously noted or not, though I would surely imagine I'm not the only one who has experienced and taken note of this.

I'm trying to get my vaults to 100% happiness at the moment, I will try one using the radio station and one without, and let you know how it goes.