Could you please share a screenshot of your flow's configuration?
Do you want to edit the text that appears above the comment box that the Approver fills using Microsoft Flow?
I have made a test on my side and don't have the issue that you mentioned. My flow's configuration as below:
The flow works successfully as below:
Please check if the Approver has filled values within Comment box:
In addition, if you want to edit the text that appears above the comment box that the Approver fills in within your flow, you could consider take a try to use the WDL expression to edit the text within your flow:
But if you want to edit that approval form in any way to include a few extra fields, I afraid that there is no way to achieve your needs in Microsoft Flow currently.
More details about the WDL expression within Microsoft Flow, please check the following article: