I guess this is the same issue, that I reported one year ago and that‘s still not solved for me: ERG problems related to slope mode - #22 by Alex
When the workout starts with a ramp, there‘s no problem. But when it starts with a steady effort, I have to press the „+“ button a few times or change to slope and back to ERG until ERG kicks in. No problems during the workout. It‘s always during the first minute.
Glad that others have the issue now, too 
TACX Neo 1, iPad