I was concerned at entering BR without an exit stamp in my UK passport from my previous trip. An unpleasant interrogation and the threat of a large fine had resulted from a similar situation in the US. But there were no questions at GRU.

In your case, I don't believe anyone here is qualified to give a definitive answer, unless we have a lurker from the BR border agency. I suppose a useful option could be to renew the BR passport. Always handy to have two...

You might find out locally if BR citizens with dual-nationality are permitted to use non-BR documentation to enter/leave Brasil. I believe that is a big no-no for some states.

Last year BR tightened up its passport and border security in order to maintain a visa-free environment with some countries, including UK and EU. That might mean it's better to play things by the rules.