Optimum's own Samsung boxes handle switched digital video fine, and I don't believe either of those channels you mentioned are switched regardless. I would be suspect of a signal issue. If you wanted certainty, you could try bringing the HDMI box that works with something like a computer monitor to the point where the coaxial cable connection is for the MythTV box, and then see if the channels tune at that location.
If you have an older ARRIS modem on Optimum, you can get signal levels (for the internet) at
http://192.168.100.1 in a web browser. Unfortunately the newer Optimum gateways by Ubee do not provide signal level information.It's not 100% 1:1 because you may have fewer/different set of splitters and in home wiring, but if the signal levels are marginal on the modem, it's very possible that your drop from the street is bad.
My parents had issues with a tuning adapter and certain channels/internet hiccups on Spectrum, finally got a truck roll and a tech, the tech took a measurement first at the wall plate (not fully tightened, fixed that for free), then at the splitter, then the drop (the cable coming into the house), signal still bad. He then went up the pole and measured the drop from the pole to the ground, that cable was bad. About 20 minutes later and the signal levels were
insanely good - actually so hot that I had to switch them from an actively amplified 5 port splitter (no loss either direction) to a non-amplified splitter that lost a few dB on every output.
(Counterintuitively, it is possible for your signal to be too hot, and it can cause issues similar to weak signal for cable internet. For downstream you want your signal to be between +/- 7 dBmV, with 0dBmV being ideal.)
Optimum hates truck rolls and will threaten you with an $80 charge if it turns out being from wiring in your home. I used to use the modem's diagnostics page as a way to get certainty on that, now my Ubee gateway lacks this.
Actually doing some research,
you can access a diagnostic page on the Samsung cable box. Note that in this example image, the Forward data channel (Optimum backend -> Samsung STB) is yellow for non-ideal signal, and the inbound quality is red, indicating that the strength of the channel on that frequency is bad enough where it's likely to impact operation of the STB.
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