I was playing a survival world with my brother when I came across a ruined portal. It looked completely fine, however a spruce tree had spawned inside of it. When I tried breaking the tree, some of the blocks' sounds and breaking particles were obsidian or netherrack. Also, the chest that had spawned has properties of netherrack also, and so did some of the frame itself. Luckily, when placing blocks nearby the surrounding blocks updated to be the blocks the properties actually were, but it can be confusing if you have just rebuilt a portal and some of the blocks were actually netherrack.