Joplin ignores iOS

iOS provides a setting which allows or denies apps to refresh in the background, over wifi, or LTE. Joplin does not only seem to ignore this setting, it is able to refresh in the background even when this isn't allowed to any app at all.
Feature or bug ?

Joplin does not have a background sync on mobile devices. When Joplin is closed, sent to the background or the device is put into sleep (display off), the sync is interrupted.

This is not what I see happening on my iPad. It keeps happily syncing while in the background, there is no doubt about that.

for iOS, there exist so many popular note taking apps, including Bear, notability, goodnotes, Evernote,OneNote and etc.

Welcome to the forum!

Of the apps you mentioned, only two can be rated as cross-platform. And none of them can cope with the level of privacy which Joplin's end-to-end-encryption can guarantee. The one thing that is lacking (and that's a big one IMHO): Joplin for mobile unfortunately has no background sync which makes quick note taking almost impossible.