I've created an issue log in a SharePoint list that contains around 16 fields, from that list I have created a PowerApp form but only brought in 6 fields as these are the ones I want an end user to complete with details of the issue, the other 10 fields are for the project team to complete with fields such as Status, Priority etc. Some of these project team fields are mandatory as I want to ensure the project team complete them, but in PowerApps it won't let you submit the form as there are mandatory fields in the SharePoint list that I don't want to include in the PowerApp form.
It is sure that you cannot leave these mandatory fields empty when submitting one item. It will cause error. But you can set those fields with some default values, like "**", then make those field invisible in the edit form. Like: