issue to ping outside gateway

Hi erveryone,

I’m asking a question today I have an issue with my CCR2004-1G with routerOS 13.5 (test wtih 7.12.2 and 7.14.3 too).
I recieve a dhcp address from my ISP in a /31network (they use IPv4 CG-NAT), the default route to the gateway and to 0.0.0.0/0 are created by routerOS.
A masquerade rule is also created for dst adress 0.0.0.0/0 with the out interface = wan

but when I try to ping 8.8.8.8 I get that :
[admin@MikroTik] > ping 8.8.8.8
SEQ HOST SIZE TTL TIME STATUS
0 22 (Invalid argument)
1 22 (Invalid argument)
2 22 (Invalid argument)
sent=3 received=0 packet-loss=100%

and If add the interface :
[admin@MikroTik] > ping 8.8.8.8 interface=sfp-sfpplus1
SEQ HOST SIZE TTL TIME STATUS
0 8.8.8.8 56 118 11ms786us
1 8.8.8.8 56 118 11ms833us
2 8.8.8.8 56 118 11ms763us
sent=3 received=3 packet-loss=0% min-rtt=11ms763us avg-rtt=11ms794us max-rtt=11ms833us

so I a little bit lost to find the correct parameter to get my network working and reach internet from the router.

just an additional info for the ping without the interface not traffic is viewable on the NAT rule