Looks like there is a bug in setMaximumConcurrentRequests method of io.undertow.server.handlers.RequestLimit class.
I set new maximum by setMaximumConcurrentRequests, then try retrieve value by getMaximumConcurrentRequests but get 0.

Looks like the bug in following line:
newVal = current | newMax & 0xFFFFFFFFL << 32L;

Should be:
newVal = current | ((newMax & 0xFFFFFFFFL) << 32L);