Hi,
Are you using SNAT? Is you are using SNAT, the client session is NATed behind this SNAT address, so the self IP can use the same port 1111 because this is a different IP/port combination.
If you are not using SNAT but the server side connection uses the IP address of the egress interface, I think the F5 attemps to preserve the source port, but selects a different port if that source port is in use. See:
K13433: Configuring source port preservation for SNATs
Preserve
Specifies that the system attempts to preserve the value configured for the source port unless the source port from a particular SNAT is already in use, in which case the system uses a different port. Preserve is the default behavior.
Regards,
Martijn.