When there's a block above the wolf, the wolf - tamed or not, won't be able to see skeletons standing right next to them, while the wolf is in the minecart. It takes having a partial block over them with the opening toward the skeleton for the wolf to see and attack the skeleton. I tried glass in the picture below, but still to no avail.
In previous versions, the wolves have no issue at all with line of sight considering they were able to see the skeleton's feet.
I can confirm this. The tallest block I have been able to put on top of the wolves and still have them attack the skeletons were top half slabs or stairs facings away from the skeletons. This also works if the skeletons are elevated by a bottom half slab.
An unfortunate consequence of the change is that skeletons surviving the wolf attack now more easily are able to hit the wolf back.
MC-148047 seems to be the same issue.