Hi!
Yes. this is a known Apple bug where the App Store's cached age-verification state doesn't fully reset after a region change, even though your Apple Account now correctly shows Vietnam. Several users hitting this exact Singapore law requires age verification message after switching back from Singapore have reported it on Apple's own support forums.
How do I resolve age verification issue f… - Apple Community
- The device's Settings → General → Language & Region setting (separate from the Apple Account region).
- A leftover Screen Time age-verification flag, which some users found persisted even after the Apple ID region was corrected — fixed only by signing out of Media & Purchases entirely.
- An active SIM/carrier location signal — one user's Singapore-law prompt was tied to a non-domestic SIM being detected, resolved by disabling that SIM and power-cycling the phone.
- A VPN or proxy still routing traffic through Singapore.
https://www.reddit.com/r/applehelp/comments/1uam991/singapore_laws_even_though_not_in_region/
If the issue isn't resolved, please contact Official Apple Support
Thanks.
MindBlown.
