Microsoft Windows [Version 10.0.19043.1288]
DP Pro 9.8

Recently (ish i think) when dragging windows from second monitor to first, the window slips and loses it's hook to the mouse cursor, so I have to click the now partially moved window and drag it the rest of the way again.

Yes, only when Display Fusion is running. As soon as the mouse hits the edge of the monitor it catches and loses the window. I notice with or without Display Fusion running, as soon as the mouse hits the monitor edge, there is a small window snap half halo animation, I think that is what causes it? Dragging the mouse much slower on purpose and the issue goes away.

The workaround is to first single left click the window title, then click and drag the window - it then drags properly between monitors.
If you just click and drag immediately you run into problems - the window will stop dragging part way through the motion.
To elaborate on the precise steps to cause the issue:
1. Create a Title Bar Button to move a window to position it to the left monitor, e.g. at top left. Then click the button to move the window there to create the starting conditions.
2. Then click and immediate drag the window, the window then stops dragging.
Workaround is to first single left click the window title, then click it again and drag it, then it drags OK.