The unexpected behavior is seen on 2.9.0.beta8 (d716e32a32). I remember that the global notice was functioning several days ago on 2.9.0.beta7.
I thought that it may have to do with the search banner component, but disabling it changed nothing.
AI-generated summary
A user, Kuro22, reported that the global notice was not appearing on their Discourse site running version 2.9.0.beta8. After testing, it was confirmed that the issue was not reproducible on a clean install at the same commit, prompting a request for more details. Kuro22 discovered the global notice reappeared in safe mode and traced the problem to the Custom Wizard plugin, which was interfering despite no obvious connection. angus, the plugin maintainer, confirmed the plugin was removing the global notice on wizard routes and asked for a link to the affected site. After reviewing, angus fixed the issue and advised updating the plugin to the latest commit. The problem was resolved, and the global notice now displays correctly. The discussion was suggested to be moved to the Custom Wizard plugin topic for future reference.
hi!
I could not reproduce on a new site on the exact commit you mentioned; the global notice shows up correctly after being set:
Could you share more details about when/where it’s not showing for you?
Does it show in safe mode?
Hi!
The global notice didn’t show up anywhere.
Oh right I forgot the safe mode before. I just entered safe-mode under various condition set, the global notice shows up. I managed to figure out that it’s the custom wizard plugin interfering. Disabling the wizard plugin brings my global notice back.
Thank you for shedding light on the issue. Though I can’t imagine how can this plugin have to interfere with the global notice… 
@angus Could you take a look at this sometime?
Yes, the custom wizard plugin removes the global notice, but only on wizard routes. Could you just link me to your site so I can see if I can reproduce the issue.
(these posts can be moved to the custom wizard plugin topic)
Ok, I will pm you the link.
Ah I see the issue. Now fixed. Please update the plugin to the latest commit.
Sorry for the inconvenience!