Overlapping Port Forward Destination on Single Source?

im.thatoneguy

For the purposes of 'simplicity'...
I have a service running on port 5555

I have two external clients:

Employee 1: 10.1.0.100
Employee 2: 20.1.0.200

I have one external IP:
30.1.0.100 (CARP)

I have two internal resources:

System 1: 192.168.1.100
System 2: 192.168.1.200

I am trying to NAT:
Source: 10.1.0.100:Any -> Destination: 30.1.0.100:5555
Translated IP: 192.168.1.100:5555

Source: 20.1.0.200:Any -> Destination: 30.1.0.100:5555
Translated IP: 192.168.1.200:5555

If I set the Destination to "WAN Address" it works. If I try to set it to "WAN CARP: 30.1.0.100" it says it can't do it because of overlapping destination addresses. Why can I use the WAN Network or WAN Address but not the actual WAN's CARP address? Is that a bug or is my logic broken?

Since both use different Source IPs for the filter there should never be a conflict between the rules right?