Set-time via timedatectl gets overwritten although ntp is set to false

Disabling the ntp and manually setting the time on my raspberry (stretch lite) with a RTC
works fine.

$ sudo timedatectl set-ntp false
$ sudo timedatectl set-time "2020-11-29 10:00:00"
$ timedatectl

      Local time: Sun 2020-11-29 09:01:37 CET
  Universal time: Sun 2020-11-29 09:01:37 UTC
        RTC time: Sun 2020-11-29 09:01:37
       Time zone: Europe/Berlin (CET, +0100)
 Network time on: no
NTP synchronized: no
 RTC in local TZ: no

But after a few minutes the time get synced again. But why?

$ timedatectl

      Local time: Sun 2020-11-29 13:37:34 CET
  Universal time: Sun 2020-11-29 12:37:34 UTC
        RTC time: Sun 2020-11-29 09:04:25
       Time zone: Europe/Berlin (CET, +0100)
 Network time on: no
NTP synchronized: no
 RTC in local TZ: no

I found this similar question but I do not use VM nor do I see which other service could be responsible for this.