RCS in Messages in iOS 18.0 does not work when connected to VPN

I recently upgraded to iOS 18.0 on my iPhone 12 Pro in order to get access to RCS for better chats with Android users. I noticed that it wasn't working (it was saying "Text Message - SMS" in all of the chats with Android users). I am usually connected with Proton VPN 24x7 and that sometimes blocks things so I tried disconnecting from Proton VPN and noticed this fixed the issue. I was able to send and receive RCS chat messages.

After doing further research, I noticed an app developer on the Apple Developer Forums and also Mozilla talking about their Mozilla VPN product reporting similar issues with RCS on iOS 18 when connected to a VPN.

Given my experience as well as these other two findings, it seems that Apple has to fix something related to RCS support while an iOS device is using VPN.

On VPN:

Off VPN: