I'm not exactly sure what triggers this, but it's been happening to me constantly lately. I'll create a list of all caches within a certain radius of a point (with no limit on the number of caches), and invariably it'll be trimmed down to the 500 closest caches after some time has passed. This persists through refresh, so I actually have to delete the list to fix it.
Added a picture of what I mean. These are the same list, but more than half the caches are gone.
It's by design... Live map is trimmed to 300 caches (similar to official app) and Search lists are trimmed to 500 caches. Both are to minimize overloading and slowing down the map.
Even 1000 is fairly usable on my S21 Ultra. I'd prefer if setting maximum geocaches to 0 on a list was uncapped (maybe with a warning?). That's what I think the behavior has been like before I think. Can't recall having seen the trimming in the past.
The trimming code was always there, but due to another issue it wasn't triggered properly until I "fixed" it. I'll look into a setting so you can adjust it higher. I've had complaints before about the time to render map markers so a cap curbs it for more novice users.