SCR 50AXE NAT rule IP addresses

Yes I think this is a limitation that the high end models have with 'Allow remote IP' aka 'Source IP' on like FLEX H how ever you should have been able to do many multiple rules so that looks like a coded logic in thinking you can't limitation when you can the FLEX H models can do this in like rule order logic like

rule1
Source IP 2.2.2.2
External IP WAN
Internal IP 192.168.0.100
External Port port 80
Internal Port port 80

rule2
Source IP 3.3.3.3
External IP WAN
Internal IP 192.168.0.200
External Port port 80
Internal Port port 80

rule3
Source IP any
External IP WAN
Internal IP 192.168.0.50
External Port port 80
Internal Port port 80

so with the above traffic does a match if its from Source IP aka Allow remote IP 2.2.2.2 or 3.3.3.3 before matching rule 3

You can try to put in a support request to see if they can match NAT logic of like FLEX H models

your only other option are get a FLEX H