Fitbit on wrist interfere with Bluetooth HRM strap?

1st post.
Hi had loads of problems with poor game performance which I resolved by ditching broadband and getting fibre optic.
However, a week ago I bought a Mi 4 fitness band, which I wear during the day but mainly use for sleep monitoring.
I have noticed that the performance of my Cardiosport HRM has been erratic on Zwift this last week when I have also been wearing the Fitbit, (but not pressing it). I have had inexplicable heart rate drop out or inaccurate reading i.e. showing 60bpm when I’m doing 3 w/kg etc.
I tried removing fitbit today, but it was still in the room on the floor.
It’s frustrating that I’ve got the Zwifting sorted with fibre optic, but now I’m getting HRM glitches.