Not typically, afaik. I don't go to the theatre much these days and never did go to West End musicals but it does seem odd. I do know the original Broadway run had a moment where one or two (more?) shows got cancelled. Can't remember if it was Covid or just winter flu but a large number of the cast had got sick, too many for them to shoo in understudies. But we're well past the winter flu season so I don't know what this could mean.
Can't recall ever hearing about something like that happening.
This just got posted an hour ago. I wonder if it's all back to normal now.