Thanks, @David_MA and apologies for my shorthand.
To test further, I built two simple test flows, with just a trigger and one action. One used the Start and Wait For Approval action, the other used Start and Wait For Approval With Text action. With the latter, you can include suggestedText as a parameter, which the approver can approve with or without modifications. (When I refer to 'the text' that's the value I mean -- not any comments added by the Approver.)
The mobile platform I tested was the Power Automate app on iOS. For both test flows, the request for Approval appears on the Approval section of that app. The flow using Wait for Approval (without text) can be approved successfully. The flow using Wait for Approval With Text offers the approve/disapprove buttons, but on submit returns the error message.
In all cases -- my original flow, plus the two test flows -- I can Approve successfully via Outlook email running on laptop or desktop. And I can Approve successfully via the Approvals tab in make.powerautomate, any browser, any platform. I have not tested Teams app on any platform.
Hope that's clearer. thanks