From all I've read it's generally a mystery on why BTA's either have bubbles or not.
That said, I'll share some personal observatinons FWIW.
I purchased a large 14"+, RBTA from a local reefer about 5 yrs ago. It had no bubble tips, and the seller said he never saw it show bubbles. I kept if for a couple of yrs in a 58G under 400w MH, never showed bubble tips for me either.
2-3yrs ago I moved it into my new 120G system ("it" was 3 RBTA's by then BTW). The new system had cycled and had LR and some corals in it for a few months before I moved the RBTA's in. Ca, alk, SG exct all were close between the tanks. The new tank was set up with TM salt, the old tank with IO.
Within hours of moving the RBTA's to the new system, I saw bubbles for the first time? The bubbles remained for many months but did gradually decline a good but over 2 yrs, though the anemones seemed to continue to thrive (10+ of them now).
I reccently added "trace elements" for the first time (I have always just relied on water changes, but I can't say I do enough of them). Within a day or so, the bubble flared way up like I hadn't seen in a yr or so.
All of my exp is purely anecdotal, but might offer something to consider.
My *GUESS* about what might be going on is that my husbandry may have been creating a long term depletion of "trace elements" or some specific trace element? Further, that moving to a different system with relatively new saltmix, and later dosing "trace elements", may both have temporarily alleviated the problem of whatever was getting depleted.
I'm in no way saying that "I've found the bubble tip secret", at all. I am throwing an idea out there to solicite feedback or maybe even stir curiosity??? My exp probably isn't generalizable, but I'd love to know if anyone has seen anything similar, or not?
jk