I’d imagine an MP will have a look at what’s going through the chamber in advance of turning up, or not as the case may be.
Whether or not they attend will depend upon whether the whips tell them to and if not, whether they feel they’re needed. In this case I imagine most thought the bill uncontroversial and as such it’s passage was assured. For that reason most would decide to get on with something that definitely did require their attention. It doesn’t mean they’re not interested or lazy.