No valid measurements could be done during those periods. Stray light, too much movement the of sensor over the skin, things like that. Wearing the watch tighter may solve it.
Actually, I find it's more common to get those gaps during sleep, because then your arm may easily rest against the mattress in a way that creates a gap between the watch and your arm. Also, during sleep your blood pressure is low, making reliable optical reading more difficult. But I don't get those gaps (with my Fenix 5+) that often, I notice them maybe 1-2 times a week.
You might actually try to wear the watch a little less tight, in case it's so tight that it affects the surface circulation.
