Some email notifications on the same issue / MR are no longer threaded properly in Gmail

Previously, Gmail groups emails by subject. It seems that they have stopped doing this and now depend on In-Reply-To to group the messages.

When notifying about a reply to a discussion in an issue or MR, we set the In-Reply-To to the discussion reference. This causes these emails to be threaded separately.

We still have the issue or MR reference in the References header but it seems like Gmail does not use this.