Clarification on whether AKS Standard loadbalancer manually choosing allocated outbound ports differs in functionality. It seems like it uses the maxnode count logic when updating from cli or through arm template but uses the current node count when updating from portal.
Based on the discussion in the comments above, it looks like when you are using Az CLI or arm template, it actually changes the AKS profile and this will calculate the max node count and validate if the number is correct. But via the portal, if a user is changing the load balancer configuration directly, that will not change the AKS profile so there won't be validation on the numbers.
I would recommend to change it using CLI or Arm template and not using portal as those would be two different operations.