So if I understand it correctly, you are saying that it creates .1 and .2 files, the first time, as expected and then when the next time it has to roll over the server.log contents into .1 file, it doesn't? Which exact version of WildFly server is this?
this is correct. I am using wildfly 10.
I can add, that when I use append = true, then serverlog increases unbound after the first iteration (creating .1 and .2).