I also find it hard to get HR up indoors compared to when outside and riding. I think the extra heat, even with fans, makes it more difficult. You can always add a few per cent on the TR if you are really nailing your workouts.
You will frequently see the opposite stated as a general rule for indoor training and the effect of heat/cooling on HR. In other words, with insufficient cooling indoors HR may track 10bpm higher
at a given wattage than the same wattage outdoors or in a cold garage with adequate fans. Before powermeters were widely available, when HR was all that we had, many coaches would caution against getting a false impression of how hard an indoor workout was, as you could pick up some "free heat beats" and more easily hit your HR target without as much intensity as was originally intended.
Similarly, if you look at the max power on climbs vs. flats threads (which also discuss max power on trainers), there is a general trend of people being unable to hit as high a power figure on the trainer vs. out on the road, whereas this guy seems to be experiencing the opposite, where his legs are cranking out big watts without an apparent commensurate HR response, and possibly low RPE as well (more info on RPE from the OP would be great).
This kind of does sound like insufficient recovery, but it's tough to say without some rest, experimentation, and RPE info.