I notice this is your first post on the Forums, so welcome to the community!
I’d like you to read over the Zwift Support article, “How to use ERG Mode in Workouts”.
The last question of the FAQ at the bottom of the article states, “I missed my interval target/stopped pedaling, and ERG mode was disabled. Why? How do I turn it back on?” I suggest reading the answer to that because your Neo should have an “ERG resume feature” which will display a message about holding your wattage to re-enable ERG. Continue pedaling, and ERG mode will re-engage for the target wattage. If that’s not happening as described in the article, check and makes sure your Tacx Neo has the latest firmware updates.
If that doesn’t help, please submit a support request if you haven’t already done so, and a Zwift team member will be happy to take a closer look at this matter.