Well, function_graph tracer is disabled on RPI2 kernel, which means this is no longer reproducible.
[root@rpi2 tracing]# uname -r
5.3.2-300.fc30.armv7hl
[root@rpi2 tracing]# cat /sys/kernel/debug/tracing/available_tracers
hwlat blk wakeup_dl wakeup_rt wakeup function nop
I'll close as insufficient data in case function_graph tracer is enabled again later.