The latest iPad Mini (6th gen) seems to have a point resolution of 1,133 x 744 (2,266 x 1,488) which is a little less than what previous iPad Minis had (1024 x 768).
Unfortunately, it appears the designer will only accept values for known resolutions and won’t accept the correct values for the latest gen.
Requesting the values be added or THR be updated to allow for custom values to be added by the user.