Yes, I get the same response when using the developer tools emulation. It is going to a separate form for mobile layout.
I am however seeing a couple of unusual errors in the console -
HTTP400: BAD REQUEST - The request could not be processed by the server due to invalid syntax.
(XHR)POST - /_api/SP.OAuth.Token/Acquire
and 0: Failed to retrieve a valid token - chunk.sp-pages-search_3d34cd4843a6b2b42453.js (1,2298)
Presume these are SP issues that you cannot help with however! Could it also be stopping the request loading the form?
EDIT- after reading a bit in Github about this, it would seem that it the request that SP Online uses to connect to MS Graph - could you ask one of your techies to take a look and see if this is the call between Plumsail Forms and MS Graph causing the issue?