I have a journey in Marketing Cloud whose basic function is to pick up subscribers who click a link in an email, hold them until a date defined by their entry source Data Extension, add a wait interval until 4pm on that date, then send them an email. I had around 3,800 contacts being held in that wait step yesterday, expected to progress to the email send step on 1st or 2nd May. However, 2,800 of them instead progressed to the email send step at 4pm yesterday.

Has anyone encountered this before, or does anyone know what the cause could be?

The Wait by Attribute is set up as follows:
Date-Based Attribute: Contact Data > Guest_Segment_Journeys > Email1Date
Wait Interval: on Email1Date at 16:00 (GMT Standard Time)

The DE field Email1Date is a date field and its value is populated by an SQL query. The value was updated at 12:16 yesterday and did not change after that. An SQL update was run about an hour later but this contact’s values did not change (neither did the rest of the 3,800 already in the list; 200 records were added).

Most of the 2,800 contacts had this value for Email1Date: 2024-05-01 09:00:00.000 (showing as 01 May 2024 09:00 in Email Studio > Subscribers)

Looking at the Journey Health, it confirms this for the wait step for an example contact:

On Email1Date:
Waiting – 2024-04-29 12:16 PM
Coming Off Wait – 2024-04-29 04:01 PM

Each contact definitely only exists once in the DE and all have Email1Dates of either 2024-05-01 09:00:00.000 or 2024-05-02 09:00:00.000.