When using desktop view, there is a hard limit on edit summary length which only allows up to 500 characters. It's a visible limit, meaning that the user can see they aren't allowed to write any more characters. On mobile view (and only on mobile - MinervaNeue on desktop functions like other skins), however, there is no hard/visible limit. Users can, for instance, insert article's content as the edit summary. When they submit their edit, it is automatically trimmed down to 500 characters (more accurately, to 497 characters followed by "..."). That should apparently fix things, but if the edit hits an AbuseFilter, there will be a problem: AbuseFilter will log the full summary inserted by the user, not the version that is shown in the history.
It may not be easy to change AbuseFilter, because there are probably other functions tied to this (tags, for example: T206490). Moreover, modifying the AF is not the ideal solution, as the user need to know there is a limit, so that they can write a better summary, similar to desktop view.
This edit was made using mobile view. The summary entered by the user is cut to 497 characters. This is the AF log for this edit. The AF has logged the full summary, while the 500-character limit has been applied to the history/final version of the summary.