T495 - random freezes, ON/OFF button but also software buttons

Recently I got a used T495 in great condition, removed the Windows SSD, installed a larger SSD and installed Linux. First Linux Mint, then Solus, and Linux Mint again. On the side: I've done that sort of thing lots of times, know about unpowering the battery before opening the laptop, know about one-sided SSDs only and such.

On all installations the laptop randomly freezes when I need to shut it down, be it via the hardware ON/OFF button or via the software: I can move the mouse pointer (using the trackpad or the trackpoint) but cannot trigger shutdowns or similar stuff. If I then do a cold shutdown by keeping the ON/OFF button pressed until the machine powers off I can work normally again. Until the laptop freezes again that is.

Any idea what I could check? RAM module?

Specs:

• Linux kernel 5.15.0-43-generic
• AMD Ryzen 5 PRO 3500U w/ Radeon Vega Mobile Gfx x 4
• 8GB RAM (but sudo inxi -Fxxx shows only 5.67GB?!)
• 2TB NVMe (Kingston SNVS2000G)