A CAN node enters the "BUS_OFF" state when the TEC value goes above 255. But if you look at the CAN specifications carefully, it has a special condition:
On Part A - page 26 OR Part B - page 63:
Start-up / Wake-up:
If during start-up only 1 node is online, and if
this node transmits some message, it will get no acknowledgment,
detect an error and repeat the message. It can become ’error passive’
but not ’bus off’ due to this reason.
In your condition, when you cut the bus, node A is the only node on the bus and hence it will remain in
Error Passive, but it will never go to the
BUS_OFF state... That's as per design.
I believe if you are the only one node on the bus, there is no point in being in the "BUS_OFF" state; rather the node should wait for other devices to start/get connected.