While exploring I came across several trees that look like they had been cut in half on the edge of a biome. I'm used to this happening in old versions of Minecraft (several years ago), but I'm unsure how common it is now. I didn't think much of it until I walked past one and started taking damage. I tried to build around the tree and the blocks I placed changed into parts of the tree that were "there," but invisible and without collision. Image 1 shows a found tree, and Image 2 shows what happens when I place dirt blocks along a cross section of the tree. Where leaves are supposed to be, the dirt turns into leaves. If I walk through an invisible part of the tree there is no collision, I take damage, and the breath meter appears (possibly only where wood is missing, cant confirm if leaves also damage/suffocate)

In image 3 i come across a mushroom tree several chunks deep into the forest biome displaying the same bug. I fill in a section with sand, then mine it out and sleep. Sleeping causes the rest of the mushroom to appear (image 4) except what I have already mined out previously.

Image 5 is a reference for where I encountered the bug. I found it in several chunks along the edge of a dense forest biome (with dark oak and mushroom trees) bordering a plains biome. In the bottom right you can see a tree displaying the bug.

PS- I was unable to find any blocks other than trees displaying this bug, but this may be because of how easy it is to spot parts of trees missing; versus it being harder to tell where dirt or stone are missing

PPS- Encountered on a realm