Behavior of Accept-Header not rfc7231 compliant?

Hi,

when using the somewhat exotic but not forbidden Accept header

Code:
Accept: */*, image/avif


on a text/html output (for example the forum homepage) Xenforo produces a 406 error.

This is not the case when using either
Code:
Accept: */*, text/html, image/avif

or
Code:
Accept: */*

which both return a 200 code as expected.

I can understand that the header combination "Accept: /, image/avif" implies that an image is expected, however referring to RFC 7231 the wildcard...