Any FTP test I'll do, will be by looking at my Power Meter head unit screen. So if I do it outdoors, while climbing a 10% hill, I'll adjust...I certainly won't try to reach a pre estimated RPM, or speed...
If you can only manage 20 km/h (for example) and use a smaller gearing and still put out 270 Watts with 3 more turns on the resistance unit at 90 rpm, it would be the same as using a bigger gearing at 55 km/h and 90 RPM, with less pressure on the tire, IF the Power output is 270 Watts, it's 270 Watts.
If you stop pedalling half a second, that half a second, you're producing 0 Watts, even if the screen on the head unit tells you otherwise, nothing to relate to the speed, or the rear wheel RPM.
Since the OP has a (otherwise very nice ) real Power meter, my take is he "feels" more resistance (wich I understand, and agree as long as we're talking same gearing, same rear wheel RPM).
And his readings are smoothened by the head unit device he uses.
I would also add this question to the OP: Check if the problematic training session was done with same cranks RPM than usual, or less ?
If the session was an easy ride, low effort, then maybe check for this:
If you have eliminated the zero on your PM calibration, this also will change your real work data. When "cruising", or when you stop pedalling, your PM should record 0 Watts (include the "0" when setting).
You won't "cheat" the real numbers this way, when spinning a low gear or resistance, and high cadence on the trainer.