You've used your DVR to record a television program with the hopes of watching the show without commercials. You turn on your TV, fire up the DVR, and start your show. As the show starts, a pre-roll ad runs that isn't part of the original broadcast. You leave the show and come back, and a different ad is played before the episode. This is new behavior and one that is not in the spirit of the DVR that you are using.
This new behavior is a beta feature being tested by TiVo on a small number of users' devices. The existence of this new "feature" came to light thanks to a post on the TiVo forum, where a user described a similar experience to the one at the top of this article. Originally, the forum responded with confusion, as no one else had seen this behavior. Eventually, a few other members said that they had experienced the same thing and were as confused as the original poster.
This new move comes just days after a report that TiVo was working on a new offering called TiVo Plus, something mentioned by CEO Dave Shull in passing, without context or details beyond saying that it "better integrates new streaming services" into the company's other offerings. It is possible that this trial could be part of that new roll-out. With a name like Plus, it suggests that there is now a lower-level offering that Plus will make better. In the case of Hulu and Hulu Plus (before the former was retired and the Plus was removed), Hulu was a free, highly ad-supported offering with content being made available later than the Plus version. Those details could easily be applied to a DVR product, with a Standard TiVo offering hosting a pre-roll ad, and a Plus version removing that advertisement.