Try clean install on some other usb/sd and try. May be you have some setting enabled that we don’t see. 
I tried clean install of CE 20/20.1/ne/ng/nightly. Maybe the problem is in used dtb…, in android (Slimbox) its ok, in Matrix its ok.
Its interesting that in revision Plus2 (X96m+, X3) 23.98 hz is ok with CE 20 ng. Problem is with revision T and with Vontar X4.
X96m+ (revision Plus2), CE 20 ng:
Did you make tests with same TV HDMI port?
Yes, Iam testing on 2 tvs, lcd and oled, both the same hdmi. Every box (rev plus2, rev T and Vontar X4) is tested on both tvs. For now X264 is always on 24 hz on X96m+ revision plus2, X265 is 23.98. (CE 20). Its so complicated 
A little progress in X3 CE ng 20.1. If I set subtitles to central europe (windows), the refresh rate is by tv detected correctly 23.97 Hz,not 24 Hz. But in X4 CE ne 20.1 is the same 24 Hz. Tomorrow I will be testing on oled tv.
If I set central europe (windows) sub coding on Vontar X4, CE 20.1 ne, Lg oled C2, frame rate of X265 coding is now correct 23.97, X264 still not, still 24.
Because I have no current need for DV, I installed CoreELEC20-ne today. First the released 20.1. Because I had problems with wired network I switched to the nightly (CoreELEC-Amlogic-ne.arm-20.1-Nexus_nightly_20230415-Generic.img.gz). Network problems solved.
In the next step I connected with the tvheadend client to the TVHeadend server on my old Wetek Play 2 (running old CoreElec 9.2.8). The public TV channels here in Germany (e.g. ZDF HD) run smoothly, but for some channels I get a strange display, see image. This occurred e.g. for the free private SDTV channels, e.g. SAT-1.
I think I have seen a similar image here on the thread, but could not find it anymore.
Any ideas?
Limit to 8 or 10 bit in display advanced settings.
@Portisch Thank you for the fast response, but it still does not work.
Under Settings/System/Display I changed ‘Force Display color depth’ to 8 bit (tried 10 bit as well), but still the same problem.
I daily use DVBViewer client on DVB-S(2) by SAT>IP with German TV. No issues here.
Maybe you try another cable or HDMI port?
@portisch I just exchanged the cable with the one from my Wetek box (and so the HDMI port), no effect. Do not know if it is a Premium High Speed HDMI Cable, but at least the one from the Wetek box is in use for a long time.
Could it has something to do with interlacing? The progressive 720p HD channels as ARD, ZDF,… are working. SD interlaced channels like Sat-1, Pro Sieben,… are showing the problem, HD interlaced channels BBC News HD, Servus TV are also showing the problem.
Display info (while playing SD interlaced channel (SAT-1)): http://ix.io/4u1F
Try 21.0 nightly as it have a lot of improvements about colour subsampling and depth.
Sorry if this isnt the right place to ask but is there any idea for a release date on CoreELEC 21-Omega release?
Just got an LG C2 and looking for Dolby Vision support.
Can’t you use nightly for now? Or you are looking for “stable” release …
Yeah, I think I’d rather just wait for the actual release rather than risk buying the hardware ( Box R 4K Plus), setting everything up and getting glitches.
Are we talking months?
Last Kodi took almost two years to go live, your TV will be almost obsolete, if we follow the industry trend. 150 bucks for the box compared with 10xmore spent on TV doesn’t look much.
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