Due to a 1.0.0 game file being used, it was possible Bowser Jr. may have crashed after each party-mode minigame for v1.1.0 game users. This is now fixed.
Another v1.0.0 game file has been updated, which may make Bowser Jr. more Wi-Fi safe. However, I cannot confirm this for myself yet.
Bowser Jr. is now playable!