Cannot manually dock floating window at base of workspace anymore

Previously (4 months or so ago) it was possible to manually dock (or tile if you prefer) multiple document windows at any location within my workspace ie. if I had two document windows open and I dragged the tab of one down to the base of the workspace (until the blue highlight line is visible) I would have the two windows one above the other, tiled in '2-up horizontal' fashion. When I have 4 or more documents open I found the flexibility of manually arranging/tiling windows wherever in the workspace really useful.

Now, for some reason, if I drag a window to the base of the workspace I do not get the little blue highlight line and am unable to dock the window at the base. Manual tiling still works as normal when arranging windows side by side, or even if you have a 2-up stack it's possible to manually position a third window in-between the two like a sandwich filling. But drag a window to the base of the stack and it can't be docked there - there is no blue line!

The only way I can arrange windows one on top of another initially is to select one of the presets (i.e. 2-up horizontal, 4-up etc.) from the Window > Arrange drop down menu. I know it sounds like a minor quibble but it's very frustrating no longer having this flexibility to tile windows manually.

Any advice would be much appreciated. Thanks in advance.