so the issue @kip is that you shouldn't be feeding a picture through croppy multiple times. Just send it through the one time when you have your final product and then if you want it for both select both from the get go.
Croppy is designed to cut Tapas images shorter because the width is larger (and we wanted to guarantee the under 2mb size limit) so croppy will return images that can be short in length as a result of this. So sending it through the tapas slicer and then refeeding that into the webtoons slicer is going to reduce the quality of your comic.
If you really want to do it separately, then you should input the original longformat picture in the second time rather than an already sliced image. I hope this makes sense.