Certain fields are defined as *required in my portal forms. However there are sc

Irrespective of what the purpose of a button is, Appian always runs the mandatory check for any field which is defined a a required field. You can address this use case in the following ways -
1) Don't make any of the fields required at the form level, instead do the check after the form is submitted (may be in the form output or a script task). Redirect to the same task if the validation fails. Make sure you have activity chained these steps.
2) If you are on 7.4, you can try using SAIL forms which offer this functionality.
3) Last option is to use JavaScript. But using JS is not recommended as forms with JS are not supported on Tempo/mobile.