When would a service not be recognized when forwarding a port on a router?

I have a Synology NAS (DSM 5.4) on which I run an OpenVPN server (on port tcp/1194, IP is 192.168.1.10). I have forwarded the port 64666 on my external IP to that address and port on my access router (a TP-Link AS1200).

The problem: I caot coect with an OpenVPN client from an internet host:

Tue Aug 23 15:05:37 2016 Attempting to establish TCP coection with [AF_INET]myip:64666 [nonblock]
Tue Aug 23 15:05:47 2016 TCP: coect to [AF_INET]myip:64666 failed, will try again in 5 seconds: Coection timed out
(...)

When ruing an nmap from within the internal network (the one the NAS is on) I get a correct reply:

C:WindowsSystem32>nmap 192.168.1.10 -p 1194 -sV

Starting Nmap 7.12 ( https://nmap.org ) at 2016-08-23 16:46 Central European Daylight Time
Nmap scan report for 192.168.1.10
Host is up (0.00s latency).
PORT     STATE SERVICE VERSION
1194/tcp open  openvpn OpenVPN
MAC Address: 00:11:32:05:92:71 (Synology Incorporated)

The same nmap from an Internet host (no firewalling):

nmap -sV myip -p 64666 -P0

Starting Nmap 7.01 ( https://nmap.org ) at 2016-08-23 15:08 UTC
Nmap scan report for myname.mydomain.blah (myip)
Host is up.
PORT      STATE    SERVICE VERSION
64666/tcp filtered unknown

In order to make sure that the forwarding and lack of firewalling is correct, I fired up a web server on a local host and forwarded the port 9999 to that server. This time the nmap from Internet goes through:

nmap -sV myip -p 9999 -P0

Starting Nmap 7.01 ( https://nmap.org ) at 2016-08-23 15:08 UTC
Nmap scan report for myname.mydomain.blah (myip)
Host is up.
PORT      STATE    SERVICE VERSION
9999/tcp open  http    WSGIServer 0.2 (Python 3.5.2)

I know that this behaviour is typical of firewalled environments, or a bad port redirect. This is why I tested on nondescript ports and only the OpenVPN one is problematic. On top of that, there is neither firewalling nor IPS on incoming (on the NAS) and outgoing traffic (on the Internet host).

Any ideas on any possible reason for such behaviour?