I have previously raised a problem with front end form and ajax submissions for a separate issue which was resolved. The problem being when you forced the form to fail, the front end form thrown an error if you try to re-submit the same form. https://support.metabox.io/topic/ajax-form-resubmission-after-failed-validation-causes-invalid-error-message/
I have recently updated to 'MB Frontend Submission v4.0.2' and there is now an issue with the same error message appearing after you try and re-submit the form (any form) without a page refresh (ajax enabled).
The shortcode has been added using the wordpress shortcode block.
[mb_frontend_form id="dc-product" post_type="products" confirmation="Updated post successfully" submit_button="Update" allow_delete="true" ajax="true"]
"Invalid request. Please try again."
screen recording here
it happens on any form that I test. I'm using WordPress 6.0.1