Change it to e.g. 10000.
I’ve checked it on my RBD52G and behaves the same way. l2mtu is set to 1598 and there’s no way to set mtu to anything larger than l2mtu.
And conducted a test: I created a new bridge and it appeared with l2mtu set to 65535. And I could set mtu to 10000 without a problem. After that I added a wifi interface to that bridge and bridge properties changed automagically: l2mtu changed to 1600 and actual-mtu changed to 1500 (while mtu remained set to 10000). If I then tried to set mtu to anything larger than l2mtu (but smaller than previously set 10000), ROS complained with “failure: could not set mtu”. If I then removed wifi interface from that bridge, the l2mtu and actual-mtu changed back to initial values (65535 and 10000).
So it seems that l2mtu of a bridge gets set to smallest l2mtu value of all slave interfaces (OP should check the whole list of interfaces, from the screenshot it’s not clear if the whole list is shown or it’s actually truncated) and can not be set. And it’s logical that MTU of bridge is set to smallest MTU value of slave interfaces … bridge (the switch-like entity, being L2) doesn’t perform fragmentation (only L3 device can do that), so bridge interface (which is actually used by router to interact with that L2 domain, it’s not used when “only” switching traffic between slave interfaces) should have MTU small enough to pass whole ethernet frame via any of its slave interfaces … because the egress physical port for a frame gets known only after frame lands on the bridge (the switch-like entity) consulting ARP tables.
Which means that OP’s setup with different MTU settings for different interfaces, enslaved to the same bridge, is logically invalid. And it could happen that if all slave interfaces got mtu set to same (jumbo) value, it would be possible to set larger MTU on bridge interface.