I have a Roamio set to use only 4 tuners due to the Cisco Cable Card firmware version not being current. Although originally Romaio recorded on all 6 tuners but when TiVo explained without the cable card firmware update the recordings could be random on the two extra tuners I disabled 2 tuners.
I have had issues with random tuners out of the 4 tuners not receiving signal. This is the same issue I have on my Premier XL4.
Last night I set the Roamio to record Dads, Brooklyn Nine and New Girl. All programs were on Fox and in order. Dads was a full recording. Brooklyn Nine was not recorded due to no signal, TiVo history gave me that information. New Girl was a full recording.
Very hard to believe this is a cable card problem or a cable company problem.