Hi Andy,
To better understand your problem, I create PowerApps from SharePoint Online >Frequent sites >Site contents >List >PowerApps >Create an app >get the list
If you can’t get your list, you can try View >Data sources >Add data source >Click Your SharePoint email account >Enter the SharPoint URL for the location of your list.
Then you can try the steps of creating PowerPoint or customize forms.
If you still can't see the field, we suggest you post a thread in the PowerApps Community as it is the specific community for PowerApps related questions.
Hope this help you.
Regards,
Eli