Dear N Wright,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and I would like to confirm whether you are the owner of the problematic Form where “Open in Excel” button is grayed out.
If yes, I would like to confirm whether you are having the same problem when you open the problematic Form in the InPrivate or Incognito window of the browser.
If you are not the owner of the Form but having co-author permissions on the problematic Form, the I would like to let you know that you will be able to find the option to download the copy but won’t be able to open the Form responses in the Excel workbook:

However, if you are the owner of the Form and even Open in Excel option is grayed out in the InPrivate or Incognito window of the browser, then I believe the problem should be further investigated by related development team via collecting HAR logs because there is not any related service incident reported at the Microsoft Service Health dashboard.
I do feel regret for not resolving the problem and redirecting you to the related development team but the fix to your concern can be find out through the HAR logs and suggest you raise a support ticket from the Office 365 admin center, where the related team will investigate the problem further by collecting the HAR logs.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia