So I would definitely recommend a breastfeeding specialist even if another health professional says there is no tongue tie.
^^^ This.
I'm sure my DS had a degree of TT, especially once he divided it a bit with his fingernail
I asked my GP about it at his 6w check and she said there was nothing "significant", but by then his weight gain had slowed and he started getting poorly and had some reflux, which could easily have been connected to the TT rather than an acid issue.
Posterior ties often can't be seen and are overlooked (sounds like what happened with Claire) and that's what DD had. Took my friend (who is a BF counsellor) a few weeks to work out what was going on after we'd tried optimising latch, biological nurturing, exaggerated latch etc
So yes, all that to say to get someone who really knows what they're doing to check and if they still think it's an issue, get a second opinion.