Hi, oikawa Shoi
This is Chaboon.
There are no particular problems with that configuration.
It doesn't matter whether the FSMO domain controller is located on-premises or in Azure.
see Below the article:
https://learn.microsoft.com/ja-jp/azure/architecture/example-scenario/identity/adds-extend-domain
(in Japanese)
If you use Azure VPN Gateway, you should configure Azure VM's MTU. see below this article:
https://learn.microsoft.com/ja-jp/azure/virtual-network/virtual-network-tcpip-performance-tuning#vpn-and-mtu
(in Japanese)