I am also experiencing the same behavior. I work in Split view nearly all the time. Pressing "o" and clicking an adjusted image would pick up the adjustments, then by simply moving the cursor over another image, the arrow would show as a down arrow where you could stamp whatever you clicked - or my favorite was holding down shift would allow a bulk change. It was super helpful, especially for dealing with portraits.
This looks to have changed in 3.1.3. I thought it was a bug, but now I'm not sure it wasn't engineered this way on purpose. The new procedure is to hold option down to flip the arrow, shift+option to bulk apply adjustments.
I have no idea why Apple would have it work one way in Browser view, and then inexplicably have it work a different way in Split view. Strange.
** If Apple reads this: put it one way or the other, please. I can adapt, but this seems incomplete, unintuitave and inefficient.