You bring up a really interesting scenario. The only reason I can think of that Zwift custom workout would change from your original design is that you’re skipping intervals, which you’re allowed to do on the fly. If you’re not skipping any parts of the workout, but it’s still altering the workout in some way, I recommend that you 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.