I think it may be related to the number of elements i add.
When i reach 25 elements, any additional elements, e.g. a free ride, causes the last step to change to 400 W. Adding a step results in a maximum power of 400 W
I have a wahoo kickr snap.
I noted that it was at 21 elements not 25 when the problem occurred.
I also edited a ZWO file manually and any entries above 21 or 22 caused Wzift to not load the workout.
Removing the extra entries from the file allowed zwift to load it
However, since the steps in the workout (looking at the ZWO file) are multiples of the FTP, then all of the lower power steps will start at a higher value.
Also, this problem only occurs when you get to about 21 elements in the workout.