Have the same problem.
We use LACP fallback in the switch so if the ports in the LACP not receive LACP one port goes into standard mode/non LACP. That's what is active now.
Trying to create a bond with the eth0 (currently used to manage) and eth1 but can't select bond mode.
Have also in the switch enabled native vlan that is the manage vlan.
LACP fallback.jpg Bond managment.jpg