Inside a multi instance loop the process engine cannot handle signals.
Here two examples of this behavior:
1. Catching intermediate event node: directly triggered

In this case the catching event nodes are active, the engine shows the waiting signals, but it does not handle the signal when they are injected in the engine.

2. Catching intermediate event node attached to an active node (human task)

In this case the nodes where the catching event are attached are active, but the engine does not list the waiting signals and it does not handle them when injected.

Not sure but it's likely linked with the following issue: https://issues.redhat.com/browse/RHPAM-2103