Actually, this is not a bug, but just the expected behaviour. As per the asymmetric multicore design philosophy, M4 is the auxiliary core to perform the low-speed low-performance system tasks when the A9 core sleeps for power saving, but when Linux on the A9 core starts, it takes all the system control on itself.
However, of course, you can manage the A9 and M4 cores interoperability as you want. For example, refer to the Section 2.6 "Remote Processor Messaging" of the attached document.
Have a great day,
Artur
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