I wasn't able to replicate the behavior you were seeing, but I've revised the code to be a little more cautious to possibly fix whatever issue you had with timing.
Update SummaryPreviewer.cs · kwsch/PKHeX@e10d272
Try the development build (wait a few minutes after this reply so that it has time to build and upload) and let me know if you continue to have issues!
https://projectpokemon.org/pkhex-dev/