FTP bug?

This was poster by someone on Facebook in the Garmin Tacx Smart Trainer group.

Trying to do a FTP test on the Nero 2T but 20 minutes (Z4) gives me a stable power to maintain like the ERG mode is still on.

I replied and got this response

… but in the 20 min test there shouldn't be a pre set power, it should just register what my power input is over that 20 min

Any suggestions; is this a misunderstanding by the original poster or a bug in the application?