It’s possible to create a tag using spaces, e.g. the fandom tag https://test.archiveofourown.org/tags/%E3%85%A4 on this work, which consists solely of the Hangul filler Unicode character. You can try copying the character here: ㅤ. (There is also a halfwidth Hangul filler character you can do this with. I am not sure if the bug is the same with the Hangul Choseong filler and/or the Hangul Jungseong filler.)
For future reference, a way to figure out what a character is based on the encoded URL:
Paste the encoded URL into this decoder and press “Decode”
Copy and paste the character into the search bar on Unicode Lookup
Upon further investigation, this appears to be a font issue. The character displays as a blank space in some fonts, e.g. the one used in the work meta (the dots are just the bottom border styling on tag links), but as a visible character in others, e.g. the one on the blurb:
Both screenshots are from macOS; on iOS, which has different fonts, both instances of the tag appear as spaces.