When creating pages in various spaces, labels are created and are uniform across projects/spaces/etc. Regardless of what labels are used, when searching by label, nothing is found ever - not for that space or any other. Essentially labels are not being recognized at all.
If the matter is related to searches not being able to recognize the labels, then I suspect that there could be something wrong with the index. Perhaps you should try rebuilding your index from scratch, and see if the problem persists.
Indeed it seems like an unexpected behavior. I suggest you to look at your Confluence Log (atlassian-confluence.log) and search for some error related to "label". If you find some ERROR or Warning related to that, you can check on https://confluence.atlassian.com for it.
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