wireless automatically gets RTF_IFSCOPE default route

I have a routing problem when using a specific wireless AP from my mabcook pro.

When I connect to the AP (wireless is configured to use dhcp) I get a default route like this.

Internet:
Destination        Gateway            Flags        Refs      Use   Netif Expire
default            192.168.20.1       UGScI           0        0     en0
127                127.0.0.1          UCS             0        0     lo0
127.0.0.1          127.0.0.1          UH              6     8360     lo0
169.254            link#4             UCS             0        0     en0
192.168.20         link#4             UCS             1        0     en0
192.168.20.1/32    link#4             UCS             1        0     en0
192.168.20.1       6c:40:aa:ad:22:fc  UHLWIir         2       24     en0    735
192.168.20.113/32  link#4             UCS             0        0     en0

Note the 'I' flag at the default route (I=RTF_IFSCOPE)

This 'I' flags prevents that the default route is used.

$ ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
ping: sendto: No route to host

when I specify the interface it is used (as expected)

$ ping -b en0 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: icmp_seq=0 ttl=58 time=9.374 ms

When I manually add a default route it all works (as expected)

$ sudo route add default 192.168.20.1

$ ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: icmp_seq=0 ttl=58 time=8.154 ms

In that case I have two default routes, one with 'I' one without.

Destination        Gateway            Flags        Refs      Use   Netif Expire
default            192.168.20.1       UGSc            0        0     en0
default            192.168.20.1       UGScI           3        6     en0

Every time I reconnect to the AP the route is set incorrectly (with the 'I') flag.

problem: why did the automatically set default route get that 'I' flag ?

other machines using this AP have no problem. (mostly windows) When using other APs, my macbook pro connects just fine. It gets a default route without the 'I'

Question: what causes my macbook pro to set that 'I' flag ?

When not using DHCP (settings the IP/netmask/gateway) manually I also get a default route with the 'I' but only on this AP. (other APs never)