Maxim, thank you!
This "bug" was a "feature" in our own proxy module based upon standard nginx proxy module. When i ported changes in 1.2 into our module i was surprised why some of our rules were not working until i found this change.
We cannot use something like
proxy_pass http://$arg_a$request_uri;
because our directives do not support this. So currently i just reverted those 2 removed lines into our module. May i expect some unwanted side effects from thу removal except those which we are using as "feature"?
Cheers, Alexey.