We have been using 3rd party email conversation for years now and always worked OK.
Since a while the subject is not showing anymore in the UI when the conversation is alerting on the UI for an agent :
while the subject is correctly set on the conversation :
You will most likely need to open a case with Genesys Customer Care. I know that a change was made recently in the UI for the interaction roster. 3rd party emails are not very commonly used, so it is possible that a test case for that specific email type slipped through testing.