Sounds like scaremongering to me. Probably hoping for lots of link bait to attract traffic to their ads. If you're showing your Residente Permanente card when you enter the country I cant see how they are going to issue you a tourist visa. It's possible they may make a mistake but writing on the form isn't going to stop a mistake being made. Whatever, even if they do make a mistake as long as you have that card I cant see how it could be revoked for their error.
The first time I ever saw mention of this was on yucalandia.com It seemed bogus to me then and still does. When a resident enters Mexico they surrender their FMM- they are not issued another until they leave again- so, it seemed illogical to me that they could be admitted "as a tourist, for 180 days" unless that's what they requested. Could someone who had gone to the pain of obtaining a resident status really want to misrepresent themselves in this way? Not likely, I would say.
This story is making its way all over the internet. Anyone with half a brain can see there is no logic behind it; unless you have yet to leave the country on a PR/TR, and gone through visiting INM at the Airport. As long as when you walk away from the counter at the airport, and you have just half of the FMM form, you are good to go.
I agree that this is probably bogus, but I can visualize the possibility that a TR or a PR could lose or misplace the FMM you turn in upon returning to Mexico, and perhaps fill out another one on the airplane. Then, if they miscommunicate with the INM agent I could see this is a possibility, albeit a rare one.
