Hi alex,
Dynamic segments calculation is not instantaneous and it get refreshed about once an hour when they are live.
While your journey is running, it will continue to process new contacts that join its segment, even if they join after the start date.
This means that new contacts can join in at any time until the end date arrives.
The Total used by this organization column in my quota limits table also not updates immediately after I created a customer journey in my test.
Time1 is 2:57 PM, it's moment after I went live a customer journey

Time2 is 3:33 PM when I opened Insights tab.(It's a service to track your contacts react)

A previously completed customer journey Insights view.

You could check Insights tab and check quota limits table again when your contacts receive emails,
and I'll give you my test result once my testing mailboxes get emails from d365.
From my previous testing, this process took up to 5-6 hours.
Regards,
Clofly