The only weird thing that happened was I noticed one week that my DX5 II wasn't going into standby mode. I thought it was because I started using the mail app on my Mac (which was left open), so the computer was simply waking up all the time because it kept getting mail notifications, so I shut the app down. However, the DX5 II still wouldn't go into standby mode, so I went into the Audio MIDI app, and saw that the DX5 II was greyed out (and I think "Unlock" was on the screen, but I honestly didn't look close enough). I believe I turned it off/back on from the switch at the back, and the device was no longer greyed out in the MIDI app.
Hasn't happened again, but there's been two FW updates since then (and I shut down the mail app). I'm also keeping a bit of a closer eye to see if it mysteriously disappears from my connected devices list the in the MIDI app again.
Now, my setup situation is a little different in that I have to have a USB DDC in between my computer and DX5 II (it fixes a popping issue from my Mac's USB output), so it could be related to that kind of connection chain, but I think what happened is very similar to what you experienced. I still maintain a USB connection with the DX5 II for FW updates, too.