FW inbound Email is not getting attached to Case although it has same ref tag

[gazalagrawa]

Detailed description

When we forward any email using FW instead of reply (RE) it is creating new case instead of attaching.

We see inbound action Update Case via Reply for EaaI for Reply email which is using flow designer for further processing and created similar Update Case via Forward for EaaI to include forward functionality but it is not working as expected. 

Agent forwards email (from his internal email) to the case that included the ref tag, but it created a new case instead of linking in the original case. Note: The subject of the email had the word “FW:” in it.

However, when he received a response from the email that was sent it was added to the original case Note: This subject did not have the word “FW:” in it.

It does have a different subject line but that could be true at times. I would expect that if an email has a ref tag associated with the case that the email will be added to that case.

How can I achieve this requirement? Do I need to make changes to Flow Designer as well? If yes where exactly kindly help me.

attached are the OOB and custom inbound action condition. I have used the same script as of OOB action inside my custom action too.