It's related to ZBXNEXT-1899.

I've tested and I can see that simple alerts, where the only difference in Subject is PROBLEM|OK first word, having the same "In-Reply-To" header are not grouped in Gmail.

But they are nicely grouped in Thunderbird, as it should !

Just FYI: What is the fun, that multiple emails (trigger multiple problems generation) having the same Subject, but sent by different events, so having different  "In-Reply-To" headers - they are grouped in Gmail, which probably not what we planned to have.

Would be nice if Gmail cold group email the same way as classic clients.

This article could explain why it happens - https://gsuiteupdates.googleblog.com/2019/03/threading-changes-in-gmail-conversation-view.html

Gmail want to use "Message-Id" header, which was considered by our devs during designing the feature, but not selected for unknown reason.