Hello,
I searched a bit more and I found out I overlooked the problem.
In em-event.php (save_events()) there is code to propagate the postmeta.
However, EM creates the events while saving the recurring event.
This means that the recurring event (ACF) postmeta are not yet saved when EM creates the events (instances of the recurring event).
I noticed that if you save the recurring event 3 times, you end up seeing your custom field in the events (provided you have configured ACF to handle it in event OR recurring-event posttype).
BUT, if you do that, you have 3 times more events than you want.
A bit tricky ??