Hi @boernard, thanks for providing a JSON export!
I believe this might be a similar issue to MomentJs not working on mobile browser on device, where input that does not follow the ISO 8601 format (YYYY-MM-DD) is causing some inconsistencies across devices/browsers.
In your case, it looks like you are setting the value to be
viewedMonth.setValue(moment(year1.selectedItem + '-' + month1.selectedItem.value + '-' + '01').toDate() )
Which would output something like 2025-1-01, but it might actually need to be 2025-01-01 in order for the date to be parsed correctly. I would try updating your select to use two-character strings for the months (01, 03, 12, etc. instead of 1, 3, 12) to see if that fixes it!