SD tuners are very sensitive to what's called Tuner Overload. Without getting deep into the RF theory, let's just say that one or more strong TV channels can overwhelm the tuners ability to see the weaker channels. When you add attenuation, which is what adding one or more splitter does, the level of the strong channel is reduced to below the critical level which the tuner can handle.
See here for more info https://www.google.com/search?q=Tuner+Overload
This behavior in the SD tuners is a direct result of the design decisions SD made. Other tuners such as those in your Channel Master DVR and TV are simply better at resisting overload.
It could also be that a nearby LTE cell site is overloading your SD tuner. LTE filters are available from Channel Master and others. Antennas with built-in LTE filters are also available from several places.
Adding attenuation and/or an LTE filter takes care of the Tuner Overload on SD tuners for most people.
Your SD tuner is probably no worse than any other SD tuner since adding attenuation helps.