Why can't I see a folder in "/storage/emulated/0" on PC through MTP? I use a banking app in my Android 8.1.0. All receipts are saved in /storage/emulated/0/APPNAME.

I can see the APPNAME folder and the receipts on Android if I use an explorer app like Google Files. But if I connect my Android on PC, the folder doesn't show up. I can see the other folders with no problem. Only that specific folder is missing.

What's going on?

(I suspect the devs marked the folder as not accessible through PC)
It seems to be a problem related to Media Providers. Due to some bugs in implementation of MTP, database of Media Storage app (/data/data/com.android.providers.media/databases/external.db) built from contents of /storage/emulated/0 isn't updated or is delayed or isn't properly communicated with the client as the files change, so MTP shows outdated / incomplete contents.

In order to force database updation, delete the file and run some media scanner app. Non-root users can delete this file by clearing data of Media Storage app.
But be aware, it'll also clear internal.db which will make ringtones unavailable and a new database will be built from /system/media on next reboot.

See this answer for more details.