Hollow tree branches are usually just a sign of a really old tree. It happens in real life too.
https://www.woodlandtrust.org.uk/blog/2023/07/ancient-hollow-trees/
https://oneilstreeservice.com/is-a-hollow-tree-dangerous-unveiling-the-secrets-within/
https://growingfruit.org/t/apple-tree-hollow-branches/25818
reddit.com/r/oddlyterrifying/comments/1ag86l7/inside_a_1000_year_old_tree/
https://www.popsci.com/millions-years-ago-hollow-trees-tore-themselves-apart-to-grow/
Only the outermost layer of a tree is actually growing. The rest of it is usually inert, and can get hollowed out by insects or fungi. Sometimes this is a damaging parasitic relationship. And other times it can be more symbiotic, with the tree actually using the insects dung and fungi as "fertilizer". It really just depends on the circumstances.
Sometimes it's a sign of a parasite or major disease. But, a lot of the time it's just a natural process for older trees.
It's possible that this could be Oda teasing that there's already something wrong with Yggdrasil. The equivalent of Nidhogg the dragon eating away at it's roots. Or it could be just a way to show us that this is indeed a very old tree. It's hard to say right now.
But, either way, I don't think this is where the final Road Poneglyph will end up. I think it's probably still with The Man Marked By Flames, most likely. But, we'll see. It's possible that we could see some sort of battle with those living "inside" and "outside" the tree. I doubt it, but it's not impossible in a story like One Piece.
