@silvia We have made a v3.1.2 release of the MQTT data source which should rectify the issues you were experiencing.
There we some bugs around referencing values inside JSON objects so we decided to switch the notation over to the JSON pointer spec. Your existing points with $. type notation will be automatically converted and hopefully should just work.
I've added a help document to the data point edit page for MQTT too (click the blue help icon).