I have had the exact same issue some time ago, and actually I was thinking this was my old thread coming up again.

What I learned at the time: what matters is how many items you are considered to have while passing the person scanning/taking your boarding pass. Past that point, it doesn't matter. As the rule is both very inflexible and also very stupid, it's easy to circumvent it. So to avoid issues I just put garment bags/clothes therein into my roller suitcase when leaving the lounge or before approaching the boarding gate, pass the gate agent, and take it out again before stepping onboard. On board the FAs usually ask me whether I would like them to hang it in the wardrobe (flying in paid F, which may make a difference).

It's indeed the inconsistency which is infuriating. The first time this happened to me was during a transfer at DFW. Arrived on a plane where bringing the garment bag was no issue, and then for the connecting flight they told me I couldn't bring it.