Strange IPv6 routing(?) issue

MODS: I'm not 100% sure if this qualifies as Enterprise Networking - My apologies if it doesn't.

Hello Redditors,

I was hoping someone could help me shed some light on the following issue:

We rent a series of VPSs from a decent provider. They give us a VPS with one public IPv4 address and a /64 IPv6 subnet per VPS.

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On this particular VPS we run VPNs, of which one is supposed to route all internet traffic for a certain group of workers. On IPv4 this runs flawlessly, but I'm experiencing some weird - what seems like split-routing - issue on IPv6.

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Here's the deal:

My outbound IPv6 traffic seems fine, when I use ping6 from the VPS itself I get a reply wherever I ping to. When a client that connects to the VPN tries to ping outside over IPv6 I can see the packets arrive at the other server in tcpdump (when I use one of the other servers to test), and I see the server sending a reply. However the client doesn't receive any response in return.

Upon investigating with tcpdump at the VPN server I noticed that the reply doesn't seem to be reaching the VPS at all. My expectation was that even if I would have some misconfigured routing rules I should at least see the reply packets arrive at the VPN servers' public IF.

Upon contacting the VPS Provider they state that according to some traceroute data the routing on their end is correct, yet the fact that I can't even observe the reply's puzzles me.

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Please keep in mind that I might be overlooking something simple, as my IPv6 knowledge isn't that great.

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Thanks for any helping effort!