Re: Sure, subjects are exactly the same
And they come from same exact address?
Did you enable app settings -> conversations -> combine on sender + subject?
If not please do and then tap "reindex conversations" two items down.
PS - just checked how the app cleans up (removes any reply prefixes) on this exact subject, the example you gave above:
"ES [1] EventSentry::12105 by Email Critical Events"
It's left untouched, as expected, so the square brackets aren't meaningful here.