If I start with @, this works fine. If not, I don’t get any autocomplete. This happens both on this site, as well as my SSO-enabled site. Android 5.0 / Chrome 41.0.2272.96.
It has been broken on Chrome for a while. The only way I can get it to work properly is by typing ‘@’ then the first few letters I know. Hit space, then delete the space and the prior letter you entered with the mention and then it pops up a dialog that gives me the chance to choose who I want.
I’ve been using that workaround for far too long (which is my fault, as I never reported it).