Tell the lender you want to be the best FHA appraiser you can be so would much appreciate them sending you a link to verify (in writing) you're wrong. That's always a hoot.
Quote ANSI or Fannie's regs if needed since you're using Fannie's form to report with. I recommend you call it whatever they want. As long as it does not affect value, who cares? Call it what you think it is. Then, if you have a lender genius on your hands and they want you to call it the green elephant bedroom, give them what they want. In your report, say "At lender request, the office bedroom is labeled as the "green elephant's bedroom. No effect on value from labeling it a green elephant's bedroom is noted in market data" and move on.
Oh snap! Wait, are elephants a protected/unmentionable class now too or just the green ones?

Report writing has become so darn hardz. The written info you desire may not even exist in FHA regs. If you have time, send FHA an email and refer to that in your report. I would just refer to it if you have it on hand but would not paste it into the report. Gov types tend to frown on pasting their email comments into a report. If you are lucky enough to receive an answer in time, just say per FHA guidance, blah, blah, blah. Hard to argue with the horse's mouth if you catch my drift.
We are not the label police, but I'm pretty sure the department of label inspections is where all the washed out USPAP police end up.
I'm so old we used to actually call the Master Bedroom, the Master Bedroom. The horror, the horror... No doubt the in-suite label (or en-suite for all you cheese eating surrender monkeys) will eventually go the way of the Master's label. Somewhere there is an official bean counter locked in a secure basement vault, hard at work toiling away coming up with the next correct PC term for that block of the new and improved future UAD 10.8 Thank goodness we have smart people covering the important stuff for us, amiright?