Now that I'm watching Roku Live TV frequently, I've noticed that in addition to normal commercials, programs are periodically interrupted by a mostly blank screen with a message to the effect of "Be Back in X Minutes" and then after an on-screen timer counts down the time, I'm returned to where the program left off. Are these brief, periodic interludes related to buffering?
No, these are not related to buffering.
These are channels that Roku leases from other sources, who also lease them to other services. The program source builds commercial spots into the programming, and puts those "fillers" in. When the commercial time has been sold, you see the commercial(s). When not, you see the filler.