You have the Location for the Tablet enabled, and Automatic Time Zone & Set Time Zone based on Location?
Other than that, I can't think of anything else off hand that would be preventing it from updating the Time Zone.
Note! In the screen shot I do NOT have my time zone set to Based on Location, BUT you SHOULD!
Open the Samsung Members app > Click on Support & Diagnostics > Run the Diagnostics for Location & see if you get any error messages?
I mean the Location accuracy would have to be off by a mile for this to be the reason, but worth a look.
Are you using a VPN? That could certainly have an affect on the Time Zone.
I have the same issue on my tab s8 wifi. Left it for a day and it didn't change automatically yet. I'm also having my lockscreen notification become icons only all the time despite changing the settings again and again. It seems to revert. Just a few bugs I've been encountering.
I have the same exact issue on my Galaxy tab s7+. Toggled off and on, restarted, still not updating. Just did it manually.