Without including examples, it’s hard to tell.
But when searching the full hierarchical path, you are likely finding an exact match as a string, so it finds it. It’s not searching the tag’s structure. It’s just matching the exact phrase as you typed it.
But when searching two words, and not finding it, you are likely experiencing what is being discussed in this thread, and the included links to similar discussions: Search documentation may be incorrect re: default AND - #4 by ariehen