Zend certified PHP/Magento developer

ISPConfig PureFTPd user can access folder linked to other site

I have a problem, where ftp users can access and modify (rwx etc, regardless of chmod) ftp folders on different sites, which is… well, not good.
I have ISPConfig 3.2.8p1, full root privileges etc. I followed the Prefect Debian10 server tutorial for the setup, which is running on a Raspberry Pi 4.

The problem in detail:
(simplified)
The main domain I use is galeen.hu.
There are more, but for this I will show only one: eternalc.tk.
They both have their own ftp users, which work just fine, without an issue.
(I won’t tell the specific usernames ofc)
mainuser for ftp://galeen.hu
eternalcuser for ftp://eternalc.tk

BUT mainuser can access ftp://eternalc.tk with full privilege (shouldn’t even log in at the first place) and eternalcuser can access ftp://galeen.hu.
I’ve been sitting in front of the monitor bc of this for 2-3 hours now. Please help :b