Dear User,
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 let you know that as per the official Microsoft KB: Poll attendees during a Teams meeting - Microsoft Support, anyone who was invited to the meeting can submit their response to poll added in the meeting.

Moreover, I have tested the behavior at my end by inviting the external users where they are able to submit the poll added to the meeting without any issue.
Since internal users can respond to the poll without any issue and the problem happens only with the external users, I have checked the service health dashboard at the Microsoft end, where there is no related incident. In this situation, I believe the problem should be further investigated by related development team via collecting HAR logs.
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