I have hit this same issue (I want a page to be titled “2008” and for its permalink to reflect that) and so far as I can tell there is no solution.
I imagine this has to do with the mere existence of date-based permalinks i.e. https://yoursite.com/2008/05/01/this-s-a-post — WordPress seems to be assuming that this is the only reason anyone would want to use a number in a permalink.
Hoping someone can prove me wrong — I haven’t looked too far into it yet, but it seems, at least on the surface, this this is what’s going on.
