What's happened here?






See post #14 for solution.

I can float mine, just running on a single laptop screen, and updated to Dorchester this morning.
 

Seems to work when I grab the bottom of the multiduck when docked to the bottom of the screen. I don't normally do it myself, so I'm not sure if it was any different before.
 

I can still float it too, either by the Undock icon in the lower right part of the screen or by dragging the bottom border as Kamikaze described.
 

The buttons for the docking options of the multidock are sometimes misplaced. They may be on top of the expand/collapse button, left of it, or missing (probably being placed too much on the right side). Usually, just opening and closing some tabs in multidock will get them to the correct place again. (This was already an issue before the D-release.)

When I have for example the Console in the Multidock and take it over to my second monitor the Multidock remains as a single bar across the track view in the first monitor and won't come over to my second monitor.

I used to have the ability to select Float Multidock but that option is no longer there.

Trying things suggested in previous post don't work. There simply isn't any option available for me to Float the Multidock.

If I roll back to Cambridge it's there go forward to the new release and bam, gone.

So it's specifically to floating on a second screen. Does it still float on your main screen. I don't have a second screen to try, but it does float on my main screen.
 
Have you tried holding CTRL, when releasing. This is meant to stop it docking when you want it to float near and edge without docking it. I may work for floating on another screen.

Mine floats on a second screen just fine.  I tested with a project that was already floated and moved to the second display and a project that was not already floating the MD.
 
S-PlatD ...

Floats on my second screen; in fact, it remembers that whereas it didn't previously.

It will only float and persist on the monitor where the track view is placed I believe. At least in Cambridge.