Writing forwarding rules only once

I've made a simple test on mininet. And it showed that controller writes forwarding rules only for the first iteration of packet exchange.

1)Start the controller on local ip(127.0.0.1:6633)
2)sudo mn --controller=remote,ip=127.0.0.1 --topo tree,3
3)login to controller ui via browser, and adding the ip getaway(10.0.0.254/24)

Then perform command in mininet:
4)dpctl dump-flows
It shows that all switch's flow-tables are empty. Then do

5)pingall
After that flow-tables are filled(may be checked via same dpctl)
doing ping for any host pair(measure time approx (I had 0.1ms))
7)dpctl del-flows
8)try to ping again the same pair - ping goes well, but time increases a lot(I got about 3.7ms) - it means that all packages forwarded through the controller
9)check again flow-tables - no rules are written(so the controller knows how to forward those packages - but it doesn't write rules)