Map Bug

Well I'm not to sure if its supposed to or not but when duel wielding a map I thought that the map would "fill out" uncovered area's with any hand. I expected that the off hand would work as the same as the main hand. However the off hand does not "fill out" new areas whereas the main hand does
Steps to Reproduce:
1. First get an empty map
2. Right click it
3. and you might want to check the different sizes (this happened with biggest size for me)