Currently if I place a 5th border wall on my nation the " Improves environment +1 " does not kick in, I tested this by setting Gov position 5 to "No position at this time." which has a 1 point negative effect on my environment.
If gov position 4 is set the 3rd option giving " 0.5 improvement to environment. " this effect can be seen, if this is a capping issue you would expect the 0.5 improvement to be having the same issues as the 1 improvement and show no effect or the 1 only give 0.5.