I received 5 coins from a faucet, then did 3 rounds of coinjoin. So in total I should have gotten 8 confirmation notifications. However, there were only 4 shown in the GUI, and the others were never there.

How should we handle more than 4 consecutive notifications?
IMO the current behavior is better than your suggestions. We can improve it by merging them, just like the coinjoin dequeue (x transaction confirmed, x coinjoins confirmed) but not important for this release IMO.
I like your idea to merge them with X transactions confirmed!
Though I do think it should be part of the release, if it is not too complex to implement, as it is the notification feature in general, which I believe should be done properly in the release.
Bring it up on the dev meeting and we decide if this work should go to the release or not.
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IMO the current behavior is better than your suggestions. We can improve it by merging them, just like the coinjoin dequeue (x transaction confirmed, x coinjoins confirmed) but not important for this release IMO.