Yes, so as I said, they have access to the SharePoint page, not the App.
If this is a custom form on a list, then the permissions are inherited from the site, but this sounds like you embedded a standalone app in a SharePoint page, which will still need the App itself to be shared with whoever wants to access it via that embed.
In that case you will need to share the APP with the same people.
Go to make.powerapps.com
click Apps along the left rail
tick your app from the list
click share in the command bar
Then type the security group or names of the people who need access to the App and click their names to add them to the list of authorised users for the App.
Click Share
