Why does apps on sdcard move to internal storage after they update?

[AC Question]

I've a HTC One m8. I have any app that allows me to, moved to the sdcard.
When an app updates it moves back to internal storage and I have to move it again to sdcard through apps manager.
Its running Android vers 5.0.1 and HTC Sense vers 6.0,

 

[B. Diddy]

Welcome to Android Central! Probably because apps always have to download and install directly to Internal Storage, after which you can manually move a portion of the app back to the SD card.