Alexa / Amazon Music: Some titles fail only on Echo devices

[pikass]

Hello,

I am running into a strange problem with Amazon Echo devices and Amazon Music in my network.
Most content works fine, but some specific titles fail only on the Echo devices, while they work on phones in the same VLAN.

Here is my setup:

Internet
   |
Fritzbox Cable 6670
   |
OPNsense (WAN: IPv4 DHCP + IPv6 DHCP)
   |
VLANs:
  VLAN10 = management
  VLAN20 = LAN
  VLAN30 = WLAN private
  VLAN40 = WLAN IoT (Echo devices)

Configuration details:

Fritzbox provides:

IPv4 via DHCP

IPv6 via RA + SLAAC

OPNsense WAN:

IPv4 DHCP

IPv6 DHCP (needed for Matter)

System → Settings → General:

DNS servers: 8.8.8.8 and 1.1.1.1


Interfaces:

All VLANs use DHCP with:

static mappings

gateway and DNS assigned via DHCP


Firewall:

Each VLAN has rules: any → any

Firewall logs show no blocked traffic (IPv4 or IPv6)


Unbound DNS:

Enabled

No blocklists

Query forwarding enabled

For troubleshooting, forwarding to system DNS servers


Special case:

VLAN40 (IoT) uses DNS = 8.8.8.8
(otherwise Alexa does not work at all)


Problem description

Voice commands on Echo devices:

Examples that WORK:

"Alexa, play 90s90s"

"Alexa, play Was ist was Junior Teil X"

"Alexa, play something"

Examples that FAIL:

"Alexa, play Spidey and his Super Friends"

Alexa responds:

"I'm sorry, I can't reach Amazon Music right now."

The same happens when entering the same request via the Alexa app (text or voice).

BUT:

The same title ("Spidey and his Super Friends")
works without problems:

in a browser on the phone

in the Amazon Music app on the phone

The phone is in the same VLAN (VLAN40) as the Echo device.


So:

Network connectivity works

DNS works in general

Amazon Music works in general

Only specific titles fail

Only on Echo devices

Only via OPNsense network

Firewall logs show nothing blocked


What I already tried

Reset Echo devices

Deregister and re-register Echo devices

Uninstall and reinstall Alexa app

Reconfigure the whole Echo setup

No improvement


Question

Why would only specific Amazon Music titles fail on Echo devices, while:

other titles work

the same titles work on phones in the same VLAN

firewall does not block anything

IPv4 and IPv6 are both working

Could this be related to:

IPv6 routing

DNS resolution / CDN selection

Unbound forwarding behavior

Geo-based Amazon Music endpoints

Has anyone seen similar behavior with Echo devices and OPNsense?

Any hints what to check next (packet capture, DNS logs, IPv6 handling)?

Additonal Information: I am from Germany.


Thank you very much for any ideas.