Thanks for these comments - appreciated.

Follow-up: I called the BKK TG Office several times, until I reached a different customer rep who passed me on to their supevisor or maybe another section who were able to attach my TK FF to the booking (I think on my fourth call). The FF is now showing up there. BUT the customer service rep said I should still make sure when I check-in at the airport that the FF is still attached. So it seems he wasn't exactly sure that it would "stick". Good reminder to keep calling until you find a rep who knows what they are doing as opposed to "there's nothing I can do" line.

I used to have specific problems with my UA number with TG and a lesser extent with BMI and Blue1 back in the day. Perhaps the fact that those airlines are no longer around is a sign for TG) I'd be interested as to why some airlines have trouble with a *A FF while others don't.