As far as I understood it from other topics like https://forum.openwrt.org/t/any-way-to-monitor-if-802-11r-is-working/73504/2?u=underworld with "k" roaming you should not have again 4 way handshakes, as the clients are already authenticated and you should see something like this auth_alg=ft. And in fact now I do not see 4 way mesages and roaming seems to work.
Regards