@Leo Johnson Thanks for posting in our Q&A.
For this issue, I have done the test in my lab. I configure "Hide the First-run experience and splash screen" and "Force synchronization of browser data and do not show the sync consent prompt" to "Enable" and assign the profile to my user group. Then I login in the Windows 10 device with my AzureAD user account and I find that I can't turn off sync in Edge.
To clarify this issue, we appreciate your help to check if there is any other policy deployed to Microsoft Edge. Please enter "Edge://policy" in Edge's URL like this:
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