Not sure what you mean by “right after". I usually don’t immediately check the tray icon after an update of the client. However, I noticed it “soon after” the update. Does that qualify as “right after"?
btw: restarting the client doesn’t help. Even clicking on “sync now" there is for a fraction of a second something happening in the lower right of the icon, but its too quick to identify.
EDIT: I tried to verify the behaviour on two macs, and to my surprise, only 3.17.3 is offered via the update button in settings. When installed, it just says its up to date. (and yes, all are on the stable channel - as always)
And, also the website seems to offer no 4.0.0 either. 
Via https://nextcloud.com/install/#desktop-files I get the download link for my desktop client: https://github.com/nextcloud-releases/desktop/releases/download/v3.17.3/Nextcloud-3.17.3.pkg which is also referring to v3.17.3
So, I try to adjust the direct link to 4.0.0: https://github.com/nextcloud-releases/desktop/releases/download/v4.0.0/Nextcloud-4.0.0.pkg
Which does in fact exist, AND… to my surprise, it has the label “Pre-release".
In fact 3.17.3 has the label “Latest".
My conclusion: I downloaded 3.17.3 again, to downgrade. (Note: Somehow I had to manually delete the v4.0.0 client. Though the installer for 3.17.3 claimed success, v4.0.0 remained.)
I assume they noticed the 4.0.0 is not yet ready, or maybe it was accidentally labeled latest such that it popped up as “there is a new version" and I just installed it as usual.
Lesson learned: Don’t install a major version step if its still a .0.0
tldr: Remove 4.0.0 and install the latest version (3.17.3) from the website again, the the tray icon behaviour is again as it used to be. (Status: 23. October 2025)