How do I resolve the Source IP address for a EXEC CICS WEB OPEN?

CICS using the LPAR's default/source IP address instead of the CICS Regions IP address is normal CICS processing. As far I am aware of, from my knowledge and subsequent research, there are no parameters within CICS that would force it to use a specific source IP address for outbound connections. What you aim to do may be possible via some TCPIP settings however.

"Source IP address selection"

"How TCP/IP selects a source IP address"

In these knowledge center entries you can see the selection sequence that z/OS TCPIP uses to determine the Source IP address that would be used for outbound connections.

Option 6 in that selection sequence has a parameter that I think can be of use. This is the SRCIP parameter which can be specified in TCPIP's PROFILE dataset. SRCIP allows the selection of a source IP address for outbound TCP connections by jobname. In this way you can ensure that CICS outbound connections use a fixed defined source IP address.

You can see more about the SRCIP parameter here

So you would specify something like:

SRCIP JOBNAME CICSRGNA 192.XXX.XXX.XX ENDSRCIP

To ensure that CICSRGNA uses a specific IP address on its outbound connection. But there is no way to do that within CICS itself.