So, vlan filttering ?
Yes. It seems that nothing solves data leakage between VLANs. Wireshark shows receipt of ICMP type 134 (Router Advertisement) packets from all VLANs. Unfortunately the developers consider the bug in the mvebu switch fixed, although my case clearly demonstrates otherwise. Therefore, I believe that I should not expect a correction in the future, and so my options are to keep this WRT1900AC in version 22.03.2, or purchase a new device.
But it is not enabled.
Let's assume that the bug in the mvebu switch is fixed as of version 23.05.0. So I'm very stupid, and I'm doing something wrong, as I see ICMPv6 type 134 packets with prefixes from other VLANs on an untagged port. How should I fix it?
I solved my problem! I abandoned my WRT1900AC and bought a TP-Link Archer AX23. This device also uses the DSA switch. I replicated exactly the same settings and everything works perfectly! I no longer see Router Advertisements from other VLANs, which confirms that there is still a problem with the mvebu target.