If Auto Scroll is enabled, event track can't trigger, so events may not fire. The frame numbers appear to be duplicated. Especially noticeable when the [View Range End Time] is small value (Working Auto Scroll).
Expect:
Custom events always fire and output log every second.
Result:
Some custom event can't trigger.
Reproduced when play sequencer after simulate attached project.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-87587 in the post.