I used HA 4200as version: 11.6.0. But, somtime Big IP system have failed HA when one BigIP has automatic reboot: one BigIP changed from active to standby and eachother no changed from standby and actvice.
I read from log LTM, I see "Failed to allocate memory for size 304 at framework/MCPProcessor.cpp:931".
First upgrade to a supported version for example 14.1.4 is not something that even the F5 TAC will check (they will tell you to upgrade first). After that if you still have the issue you may open a case as it couldf be for device RMA: