Custom navBar style when calling Actions.pop()

Hello everyone,

I apologize in advance if this issue already exists, but I haven't found it.
I'm using the dependencies below

"react-native": "^0.59.0"
"react": "^16.6.3",
"react-native-router-flux": "4.1.0-beta.4",

I've been using custom navbars in almost all of my scenes, and in most of them I've got a back button on the left, that calls "Actions.pop()". The problem is as soon as I call the pop method, during this transition time, the navbar becomes the default one (With the blue back button on iOS).
I've got no idea how to fix this issue since I really didn't have this problem before upgrading react native and other libraries. This could not even be related to the router-flux.

Versions before upgrading:

"react-native": "0.55.4",
"react": "16.3.1",
"react-native-router-flux": "^4.0.0-beta.31",

Help would be really appreciated.
Thanks!