What keeps modifying my /etc/group?

I am trying to add my user to several groups. I used sudo usermod -G group1,group2 to add my user to the supplemental groups I had created myself. My username shows up in /etc/group then. However after reboot they are gone. The groups remain in /etc/group but my username is removed. Furthermore id yields a supplemental group (let's name it group3). In /etc/group I'm not a member of it. Grepping over etc yielded files named /etc/group- and /etc/gshadow- containing my membership of group3. What's /etc/group- and why are my supplemental group memberships removed from /etc/group after reboot? I am using Void Linux, and:

$ uname -a 
Linux ojiisan 6.6.52_1 #1 SMP PREEMPT_DYNAMIC Sat Sep 21 15:47:36 UTC 2024 x86_64 GNU/Linux

The GUI is Gnome.

[ariser@ojiisan ~]$ grep group /etc/nsswitch.conf
group: 		files