I don't know, but the experiment provides 100% confidence in the OpenWrt configuration. The problem is the server host, not OpenWrt. I'd recommend that you investigate all the config settings on the host -- from the host's local firewall and other security services (including antivirus software) to the configuration of the services themselves and their configurations (do they allow or prohibit connections from different subnets). It could be something with docker (as a layer) and/or with the container itself.
But, with that in mind, the one thing we can conclude is that port forwarding is working as expected on the OpenWrt side.
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Thanks! 