What I did:
Open the Timeline section in Agenda.
What happened:
There are some reminders, due for tomorrow. Initially they are shown (1st image). But when I scroll the timeline to another date, and scroll back to the original date, these reminders are no longer shown (until I restart Agenda) (2nd image).
Are these repeating reminders?
I have a couple of repeating ones, but the others are regular reminders.
Having said that, today it seems to be working fine - if I scroll around and come back, they are shown correctly 
In case it happens again I will get back.
One thing you might try is viewing the reminders in GoodTask. Specifically, check if the start date is different from the due date.
It appears to me that Reminders have two bits of data - start date and due date, but neither Reminders app nor Agenda shows start date or makes it editable. But whenever I have a Reminder that’s not showing up properly in Agenda, it turns out to have a different start date than end date.
It happens with repeated reminders. I haven’t fully figured out the pattern. I think it may be related to rescheduling / skipping / “remind me an hour from now” (deferring) etc. Or it just gets out of sync after a while (e.g. a weekly repeating reminder that I have for months / years, like take out the trash).
The only “fix” I’ve found is to have a GoodTask action to clear the start date from a task, and periodically clear any that need it.
