What triggers the edit can't be applied problem when editing your nominations?

Back in the early days of Ingress' "Operation Portal Recon", you couldn't edit your submissions once they had been submitted. Now we have Wayfarer, and your nominations can be edited - at least as long as they are still in the "queue" and have not yet gone into "voting" (in my opinion, these would be better be labelled "pending" and "voting", but I digress).
However, if I try and edit my nominations/submissions, often I get an error saying my edit can't be applied, and I have never been able to figure out what causes this. It's not age - yesterday I successfully edit a submission 995 days old (that got an upgrade immediately afterwards, and now, one day later, has already been turned into a portal); but some other submissions from the same time I couldn't edit successfully.
It's also not anything to do with numbers of characters - which are now helpfully shown to you. Nor is it to do with copy-and-pasting versus manually typing your edits.
What causes some edits to be disallowed, and some to go through?
Comments
Are you editing with some words that aren’t allowed? I know for a while (not sure if still correct) that people couldn’t nominate Victorian Stink Pipes (whether they’re actually good candidates or not is a separate discussion) as Wayfarer & Ingress/Pokémon GO would not allow the word “pipe”.
Others reported that they could not use the word “Nene” in their nominations, and there’s a river in England (and various Waypoints to do with the river) called River Nene so that caused some issues too.
It may be due to that?
It *could be*, but I doubt it, because of the discrepancy in frequency. In my experience, less than 1% of nominations have a problem with banned words/characters, but I can only edit nominations about 50% of the time. I wouldn't expect a different list of banned characters/words for nominating and editing.
(The banned list is one of the reasons I write descriptions in English rather than Japanese - at least I have some inkling of what may be the problem in English, but no idea in Japanese. Even my Japanese friends can't understand what the problem is when they meet a banned character).
I've gotten the error saying my edit can't be applied... and when I went back the next day, the edit had been applied anyway.
Other times, I've gotten the error, saved off the text... and the next day, the exact same text was accepted.
It acts like some kind of server overload. Maybe the upgrades currently in progress will fix it?