Hmm, I just tried moving an app that I knew had an update to SD on my tablet (Galaxy Note 10.1 2014). I could still update it just fine while it was on SD, but I noticed that when I went into the application manager it now had the "move to SD card" button back, as if updating had moved it back to the device, or the update had been installed to internal storage regardless of where it should have been.
That's not how things used to work under Gingerbread, so I have to suspect this is a bug in the way that Samsung re-implemented this feature after it was dropped by Google. So it seems that my situation is not so different from yours: I could do the update, but then I'd have to move the app again anyway (and hope it's not just wasting space on the SD with an older copy).