Thanks Mary.
Tracker created https://tracker.moodle.org/browse/MDL-41263. I tried copying the css into the custom css that you suggested and it made no difference. I did purge caches etc.
Bit of a weird one this, the lack of pop up after day 1 of a consecutive day event is a real pain for things like school summer holidays, where the whole of August say has the event 'summer holiday' in but no info on mouseover, so it appears that there is no calendar entry.