I’ve been wondering the same thing about the charging limit and its impact. The issue started for me with my older iPhone when the release candidate of iOS 18 came out.
Clearly, something isn’t right on Apple’s end. The idle battery drain is excessive. This morning, I unplugged my phone at 9 a.m., and it held at 95% (my charging limit) for only 20 minutes before starting to drop rapidly. Every five minutes, I lost a percentage until I finally picked it up to use it. Now it seems to have stabilized a bit, but I’m really puzzled by this behavior.