Proberly it is not the Makemkv software that is the problem, I actually don't know, but I have posted the problem on Plex forum, send them logfiles, and got no answers, I have posted it in Google helpsites, and get no help. So now I'm trying here, and also will try at the Jellyfin forum.
Long story short: One year ago I sold my house and bought a new house, and due to lot of work, the plex server ( a PC) was not set up until recently.
In the old house I streamed to two Chromecast with google tv, and two older chromecasts+ mobilephone (android) , often to several devices at the same time.
All connected to wired cat. 6 lan, and everything worked 100% using Plex.
Now in the new house, I have tried several set ups:
Plex on wifi : it works in 720P and below and on some HD MKV (made by Makemkv from bluray). on cat.6 lan it's the same, lagging in HD and up, BUT remark I wrote "some"
Foreexample all Alien movies, Skyfall, and Terminator movies don't work, but No time to Die works, Spiderman, it's 50/50 and so on. totally some works some dont..
Therefor I re-ripped Skyfall, but that did not help. Then I reripped Alien, that also didn't help. Then I used Handbrake to create them in MP4, that also didn't help.
Then I installed Jellyfin, and to my surprize Skyfall worked, but Alien didn't.. Spiderman movies are still 50/50, but it is better, than Plex generally. and so on.. some works some dont...
Then I tried the samsung TV again, still with Plex, hence Jellyfin has no app for samsung Tizen....
The problem in the Samsung TV is: subtitles are missing in some cases.. sound is weird, sometimes it sounds like it is the rear sourround I hear in frontspeakers.. (or centersound is missing) and every aprox. 12 seconds the sound is disturbed in a millisecond.
I have tried also to reinstall Chromecast With Google TV= didn't help.
I have tried to use a Chromecast HD, didn't work better
I have tried to update TV,= it is updated.!!..
I have tried to change settings in Chromecast with Google TV from 60 hz to 50, to 24hz and several other things= nothings helps.
..I have have updated my PC ( there is NO lagging on the pc at all by the way !!)
I have tried to update the land card driver also...
The PC is an ASUS mainboard, AMD 7-2700X processor, 48Gb ram, geforce rtx2060.
Is it the windows updates that came after one year?
IS it the updates on Chromecast with Google TV ( stupid long name...) ?
On the android phone, even on mobile network only, everything works super fine !!... no problems what so ever (as usual I could say)
What do I overlook?
The question is: what should I try now? despite throwing TV and PC, Chromecasts and cables out the window, and go back to VHS

I made a youtube recording with my mobile to show how it looks on the Samsung TV using Chromecast with blah blah, and Jellyfin and Plex
https://youtu.be/fEaSqgxPHwg