Notifications in Outlook are grouped by subject if the view settings are set to "Arrange by Date (Conversations)”. If the subject is different then it doesn’t thread in Outlook.
GMail doesn’t use the subject; it probably uses the In-Reply-To header to thread conversations, so that always works even if the subject is changed.
I believe that you can customise the mail templates in the messages that get sent out such that you can harmonise the subjects. If each type (e.g. merged, created, etc.) has a different subject then they won’t thread.