Who knows, let me summarize:
- In the lounge, before boarding, the VPN worked fine (I always use a VPN on my laptop).
- On board, the wifi worked fine, without VPN.
- The moment, I start the VPN, I have traffic through the VPN for something like a minute.
- Then the VPN kicks out.
- Then the wifi works fine again (once the change of IP address has trickled down to the applications).
- In the Wing F lounge at HKG, the wifi worked fine, as well as the use of the VPN.

And I use 2 different VPN "brands": ProtonVPN and my company VPN.

The VPN type for both is OpenVPN.

So, just make up your mind.

And, yep, probably some VPN's / VPN-types may work, when these have VPN obfuscating activated by default (for example), my VPN's don't have that.

Or so to say: Don't count on the VPN working properly, when flying CX on an A350 and have alternatives available.