The latest version 6.0.0 update for Nintendo Switch has fixed the HORI D-Pad Joy-Con’s battery issues.
When the HORI D-Pad Joy-Con was first released in Japan, Nintendo Switch owners reported the controller continued to sap battery off the Switch even in Sleep Mode. Someone’s Switch had bricked as a result of leaving the controller attached for a long period of time.
Thankfully, the issue has now been fixed, so you can safely leave the controller attached to your console without worrying about any problems.
— HORI /ゲーム周辺機器のホリ (@HORI__OFFICIAL) September 19, 2018