I was looking back at old hairstyle of the medieval period, and something that strikes me is that amongst some Germanic and Gothic peoples you often see stuff like knots of hair or straight up long hair (like with Merovingian kings), but all that stuff seems to have one big common issue : how the hell do you wear a helmet with these ?

In some cases, like especially in the case of the Suebian knot, it seems like it would get in the way of putting anything on your head, let alone a helmet. I can understand that most people may be fighting bare-headed, but for the helmeted bunch it seems like a strong impediment.
Even if they untied the knots before getting ready, that would still leave the issue of somewhat long hair. Do they bunch it up under the helmet ? Or would they let it flow behind the helmets ?
In that later case, which seems more practical, even before the age of plates that sounds like it'd get caught on stuff like mail shirts and all.

Have you guys ever seen anything in ancient texts, or tested stuff in HEMA to see how it could work ?

I believe in the case of the Swabian knot, they didn’t wear helmets. At least, in artwork they are depicted in battle without helmets. As far as long hair. One can wear a helmet with long hair. Reenactor and youtuber, Metatron has long hair and wears helmets. He may have even made a video about it.