This.
I don’t know about others but the only reason I put both is so that whichever someone clicks on, they will find my fic. So if there is supposed to be rules, I guarantee you that no writer knows these ones. We can barely get people to comment, you think we’re going to specifically choose & or / ? Hell no.
I’ve been in fandom for twenty years, and “/” means romance and “&” means no romance was literally one of the first things I learned. It dates back to Star Trek fanfiction of the 70s. I’m boggled by the fact that anyone who’s been reading fic on AO3 for more than like five minutes wouldn’t know that, and I’m curious as to what fanfic community you come out of.
I don’t think that tagging with both is actually going to get your fic in front of more readers. People looking for romance often exclude the “&” tag if there are too many gen fics tagged with both. People looking for gen often exclude the “/” tag if there are too many fics with both. So rather than putting your fic in front of twice the people, you are in fact more likely to get your target audience ignoring your fic because it has a tag they don’t want.
Also, by overtagging you are more likely to annoy potential readers away from your fic than entice them. A fic tagged both & and / better have both romance and a ton of platonic interaction between the two characters, like a slow burn romance friends-to-lovers arc. If it isn’t, I’m going to be very unhappy because the author lied to me with the tags to try and trick me into reading a fic with deceptive advertising.
When I’m in a fandom and see tagging where some of the tags don’t really apply and are just there to get it in front of more eyes, I’m going to assume one of two things. Either the author is a newb who doesn’t know anything, or the author is purposefully spamming the tags because they don’t care about lying to their potential audience and think that “spray and pray” is an effective tactic. In the first case, their writing probably will not be very good, so why bother reading their fic. In the second case, the fact that I can’t trust the tags to be accurate means I’m not going to read it to see if it’s interesting even if it has a tag I like. Chances are, that tag isn’t actually in the fic anyway, and even if it is, by spam-tagging the author is making the archive harder to use for everybody. Why would I reward bad behavior with attention? No. Far better to mute the author and move on.