As per the discussion under https://issues.redhat.com/browse/UNDERTOW-2512, UndertowHttpHandlerAdapter needs to start async handling explicitly by calling dispatch.
Comment From: philwebb
Will this one be backported as some point?
Comment From: simonbasle
The plan is to backport it to 6.1.x at some point indeed. We want to be a tiny bit more cautious with upgrading the Undertow baseline in the existing 6.1.x line, and delaying the backport could be an opportunity to gather feedback on the change from 6.2.
Comment From: simonbasle
@philwebb this has now been backported to 6.1.x, available in 6.1.16 snapshots :+1: