TV turns down the volume on its own

Hello. As the title says, I have an issue with my TV. It turns down the volume on it's own.

It's not an issue with the remote control, because I have 2 (for the TV itself and a Fire TV remote), and neither can turn the volume up/if I turn the volume up with the remote it just turns immediately down again. Also, if I try to turn the volume up at the TV directly: same result.

We used to have some issues with the TV randomly dialing the volume UP, but I always thought that was some interference from neighbors remote controls, or something similar. It's also never held on for over 24h, which is the problem now.

I would add a video how I turn the volume up and the TV actively fighting against it, but I can't add media😅

Maybe someone can help me? I would really appreciate it!

Model: Telefunken T40R906 DVB-CT

It's from 2012 and I got it from a friend, so it's quite old, but aside from HDMI 1 everything works w.o issues.