@PhilHDemo Are using any classic experience column in your list like Location/Geolocation, etc.?
Using classic experience columns might force list forms to open in classic experience.
Also, did you activate SharePoint OOTB approval flow or using power automate approvals? If SharePoint OOTB flow, check if this feature added any columns in list.
Also make sure Modern experience is selected for your list from List settings > Advanced settings.
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