Ruleflows are not working correctly in Stateless session

We are currently migrating from 5.0.1 to 5.1.1. and our RuleFlows are not working anymore. After some investigations it seems it could be related to the knowledgesession type (Statefull / Stateless). When launching a ruleflow in stateless session, it seems that the process stops it's execution after the first RuleFlowGroup found in the flow (maybe after some other node types ), and that without any reason nor exception/error trace.

In attach, I made a ruleflow sample with two nodes (one ruleflow group and one script node). The nodes are only making a simple System.out.println. When executing it in Statefull session, we have two outputs. When executing it in Stateless session, there is only one output, this one of the RuleFlowGroup. The script node has not been executed as it should be. No exceptions were found in logs.