The explanation you are looking for is here:
tl:dr Services run in session 0. Session 0 does not have access to the video driver so hardware acceleration is unavailable to PMS as a service.
AlwaysUp can be setup to avoid running Plex in Session 0. Run Any Application as a Windows Service at Boot | AlwaysUp
How to Run Plex Media Server as a Windows Service (2019/10/2016/8/2012/7/2008)
To avoid session 0 you’ll need to setup auto-logon: Frequently Asked Questions (FAQ) and Answers when Running As A Windows Service with AlwaysUp
I ran PMS as a service until I upgraded to a server that supported hardware acceleration. Since I spent about $800 building this new server, another $50 for AlwaysUp didn’t seem like a huge expense for being able to take full advantage of my new hardware. Besides, cutting the cord and getting the local channels over the air has saved me over $100 a month for the past year. I’m still money ahead.
