Hey Neomi-
What’s happening is that when you remove the headset and call EnableHMD(false) node in the blueprint, this disables input to the headset and prevents it from being registered when put back on. Adding GEngine->HMDDevice->EnableHMD(true); inside your if (GEngine->HMDDevice.IsValid()) statement will first re-enable the headset before calling OnHmdWorn().
Cheers