I currently have an issue with my iMac (also happened with the MBP) and iphone 8 (also happened with the previous iPhone 5S) that if I have bluetooth enabled on

I currently have an issue with my iMac (also happened with the MBP) and iphone 8 (also happened with the previous iPhone 5S) that if I have bluetooth enabled on my phone and unlock it... my bluetooth mouse stops responding on the iMac. Interestingly my phone and iMac are not paired and nor is the mouse and phone. Very frustrating