At exactly what point of execution are merge fields in EmailMessage resolved?

Question

I want to create a custom logic that will copy the value of EmailMessage.Subject after the merge fields used in it get resolved.

I tried to do it on the after insert trigger on EmailMessage object. However, it turns out that the merge fields used in Subject field are not always resolved before after insert trigger execution. Is it a normal behavior? Should I use a flow for this case instead?

Lightning Email Template configuration:

Here is how the email is send using Case.SendEmail action:

Here is the received email (don't look at the yellow part as it is automatically appended to email subject in the after insert trigger, it's not a merge field that is used in the email):

What I want to achieve? I want to save in some field on the Case object the latest email subject which is related to this case.

Answer 1

I've just found the solution for this scenario. I've created the Record-Triggered Flow on EmailMessage object that is ran everytime the EmailMessage is inserted and it's Status is equal to Sent. This is propably the best way to copy a value from a certain EmailMessage field in which the Merge Fields were used.