To reproduce:
1. Open a window that resizes in steps based on line-height, e.g. Terminal or Emacs.
2. Maximize the window vertically, using some assigned keyboard shortcut.
3. Press said keyboard shortcut again to restore the window to its former size.

Results in 4.8.1:
The window has its original size, as after step 1.

Results in 4.8.2:
The window fills every pixel of the available vertical space, i.e. the height of the content area is no longer a multiple of the line-height. The original size of the window has been lost.

In other words, if I open a Terminal window at 80x24, Xfwm 4.8.1 lets me toggle between that size and 80x50: 24 -> 50 -> 24 -> 50 -> ...; but in 4.8.2, after the first invocation, it only toggles between 50 and 50 plus some extra pixels: 24 -> 50 -> 50.5 -> 50 -> 50.5 -> ...