Behemot
Уверенность в том, что косячят девайсы посередине постепенно приближается к 100%
понял, спасибо, буду копать в это сторону.
Прочитал про это
Multicast Enhancement
If clients do not send IGMP (Internet Group Management Protocol) messages, then they are not registered as receivers of your multicast traffic. Using IGMP snooping, the Multicast Enhancement option isolates multicast traffic from unregistered clients and allows the device to send multicast traffic to registered clients using higher data rates. This lessens the risk of traffic overload on PtMP links and increases the reliability of multicast traffic since packets are transmitted again if the first tranmission fails. If clients do not send IGMP messages but should receive multicast traffic, then you may need to disable the Multicast Enhancement option. By default this option is enabled.
Попробую отключить и проверить.