Has anyone had the issue where list or select fields don't work in Suitelet when the Suitlet is rendered in a Portlet using an iframe?

This may not be similar, but... I just had an issue with unresponsive list fields on some records. NS status page is reporting that they had a UI issue, and recommend clearing browser cache. After doing that, fields are now responding.

Unfortunately that didn't help but thank you for responding.

does the suitelet work outside of the iframe?

yes

you can try sharing your code, but there are usually only problems with select fields if you dont return the same form object on all GETs to the suitelet

yes, there is only one get and returning one form