Power below target in ERG mode workout

For so short an interval, there is a difference between target power and "average" power. Unless the interval is longer, the average will always be lower than the target. It's not an instantaneous move from rest to interval target. It might take 5-10 seconds for the trainer to read and adjust upwards to the target.

Yeah.  In ERG mode, it's the trainer that is controlling the resistance to to try to maintain the target power.  The Garmin merely sends the target power to the trainer and the trainer does the rest.  During the interval the Garmin just records the results.  Ramp up of resistance at the start of the interval and cadence changes during the ramp up and during the interval will affect average power.  e.g., as resistance goes up at the start of the interval, you cadence may slow slightly, which means the resistance must further increase to reach the target power.  The trainer only looks at current power, not average power, so doesn't compensate by increasing power over target if actual power was previously below target.  Usually, the net result is slightly lower average power.  IMO, 268 vs a target of 276 is within reason.

In any case, IMO,  the difference is due to how the trainer regulates power and responds to cadence and target changes and not due to anything the Edge is doing.  Trainerroad may do things in controlling the trainer and in recording the data that compensates for a trainer's characteristics....perhaps like sending the target power a few seconds early to account for the trainer's response time.  IDK.

Appreciate the response. What you say makes a lot of sense and I think you might be right about trainer road and programmes like Zwift also as even with short intervals I would find the average power always matched the target power.

Dont You have dropouts in recorded power? Can You share the FIT file (in ZIP file) of this workout here ?