UPS NMC3 IPv4 Fallback not working

[LaXaRuS]

Hi,

I could not find a proper channel to report bugs so I will post this here for now.

Environment: SRT5KXLI with NMC3
NMC Firmware: v3.4.1.1

Trying to set up mail SMTP notification, I have noticed that the UPS does not fallback to IPv4 if IPv6 DNS is working and there is a valid IPv6 address.
Set up an environment with a valid ipv6 addresses (ULA) and global ipv6 dns is working however there is no route to internet via ipv6 whereas the ipv4 stack is fully working.
set up SMTP notification with Microsoft Exchange or any other mail server with dual ipv4 and ipv6 stack
UPS resolves DNS via IPv6
There is a ULA IPv6 for UPS
but no route to internet via ipv6
UPS does not fallback to ipv4 if it fails and constantly retries which indicates RFC 6555 (Happy Eyeballs) have not been implemented properly.
Please, see below capture packet.

01:19:09.780266 IP6 fd8a:4e3b:9c9c:1::120.65450 > 2a01:111:...: tcp 0
01:19:10.780155 IP6 fd8a:4e3b:9c9c:1::120.65450 > 2a01:111:...: tcp 0
01:19:12.780264 IP6 fd8a:4e3b:9c9c:1::120.65450 > 2a01:111:...: tcp 0
01:19:16.780671 IP6 fd8a:4e3b:9c9c:1::120.65450 > 2a01:111:...: tcp 0
01:19:24.781368 IP6 fd8a:4e3b:9c9c:1::120.65450 > 2a01:111:...: tcp 0
01:19:40.782480 IP6 fd8a:4e3b:9c9c:1::120.65450 > 2a01:111:...: tcp 0

The workaround is to disable ipv6 on the UPS however this will disable all local ipv6 stack.