I went to the app store and found this review
"One downside is the undocumented limitation of max 50 items when using XPATH, also when working with show more (it will simply never show more than 50). Only way to show more than 50 items is to use a Microflow as datasource. Also it's unfortunately not possible to change the default behaviour which shows selected items always before non-selected items. "
I would try using datasource as microflow to see if that fixes your issue.
I got curious about this.....if you wanted to change this in the widget, it would be a simple change. I unzipped the mpk for the widget and looked in the file SimpleCheckboxSetSelector.js. I then searched for '50' in that file. There are 2 places that number appears, the first is in the Xpath retrieve and the second is in a function that reserves space on the web page. If you changed those both to 75 or 100, rezipped the directory structure and placed that in your widgets directory, I think it would work.
Might be something you want to try out.
Hmm, did you leave the limit selector to 0 (default)? It seems like you might have set that to 50 for it to cut off like that because it won't paginate. Can you confirm?