Queue problem

Hi,

I’ve been trying to implement parent/child queues as follows.

Parent queue - 4M
child queue - LAN - 3M
child queue - wifi - 1M
whenever wifi queue is not utilized, bandwidth should be available for LAN and whenever LAN queue is not utilized, bandwidth should be available for wifi. in worst scenario, wifi should get 1M and LAN should get 3M.

my queue configuration as follows.

/queue simple
add max-limit=4M/4M name=INT_Parent queue=default/default target=“”
add limit-at=3M/3M max-limit=4M/4M name=LAN parent=INT_Parent queue=default/default target=172.16.0.0/24
add limit-at=1M/1M max-limit=4M/4M name=WiFi parent=INT_Parent queue=default/default target=192.168.150.0/24

But what happens is when LAN is fully utilizing 4M, if wifi tries to utilize bandwidth, LAN will drop below guaranteed 3M. both child queues will be having equal bandwidths 2M each.

Router RB2011UAS running RouterOS v6.39.2

Is this a bug or is there is any problem with my configuration?

Any help will be appreciated.
Thanks.