According to the HTTP2 specification - Pseudo-Header Fields, all pseudo-headers have to be together before all rest standard HTTP headers:

All pseudo-header fields MUST appear in the header block before
regular header fields. Any request or response that contains a
pseudo-header field that appears in a header block after a regular
header field MUST be treated as malformed (Section 8.1.2.6).

All requests that does not conform this requirement should be treated as malform requests. Currently Undertow does not perform any check that peer conforms this restriction and such requests are accepted.

This might not have any negative effect on functionality but if we want to be specification compliant, we should perform such check.