In general maya works by passing information between different nodes. Most of this passing is done through direct connections. But some behavior are more complex and can't be done with a direct connection. Have you ever noticed that the rotated attributes of the joints in your ik chain change, but they aren't connected to anything?
These more complex behaviors are done through callbacks. Callbacks will look for a specific thing to happen in your scene, such as a node being created, an attribute changing, or a connection being made, and when that even happens the callback runs some code. The IK system uses callbacks, it gathers the data from the joints between your start joint and your end effector (using a callback not connections), uses that data to calculate the rotations to apply, then it applies those rotations with the callback.
The ikHandle knows which ikEffector belongs to it because of that message node connection.
When you break the connection the callback stops working. This makes sense because without the connections the ikHandle wouldn't know which effector in the scene to use for the calculation
When you redo the connection the ikHandle should start working again, but something internally doesn't get updated.
If you reparent a node in the scene or create a new transform node then the ikHandle will start working again.
This seems like it's probably a bug (I'm testing on 2018 and it happens in that version). But hopefully that explanation helped you out.
