@ArcherAngelo
@Chebadoo
Hey guys. Check out page three on here. Apparently there's a glitch where several leaf piles are "overlapping" each other. Sometimes, it takes 4 or 5 tries before the leaves will actually turn to ash.
You could probably skip the burning all together with the temporary fix @Kampfkatzen discovered. Hit CTRL + SHIFT + C and in the window, type testingcheats on. You can close the window by hitting CTRL + SHIFT + C again.
Once you've done that, go into buy mode, and purchase an INDOOR trash can. Place it anywhere you want. Then while holding down the shift key, click on the trash can. You will have an option that says "Set as head". It's totally weird, but just click that.
You Sim's head will now be a trash can. (Don't worry. It comes off again.) Go to the areas where you have leaf piles that haven't gone away. Click on the leaf pile that you want to clean up, and the "Throw Away" option will now be available... since your head is now a trash can. It may take SEVERAL attempts to pick up either the ash piles (if you've burned them) or the leaf piles... because, as mentioned above, there is speculation that these piles are layering on top of one another. If it doesn't go away the first time, just click on it again to throw away. (It takes on average about five clicks when I've done it) Eventually, they do all get picked up and thrown away in your stylish trash can head.
Now, what I discovered was that the trash can head was getting "full". So, after you've cleaned up one area, you probably want to go ahead and remove the trash can from the sims head. Run back to your home lot, then hold down shift and click on the trash can that is ON THE SIM'S HEAD. From the menu, choose "Reset Object". The trash can will be removed, but may remain in the yard. You can repeat the whole process if there are several areas that need cleaned up.
Here is a link to a video on my google drive of MY Sim cleaning up some ash piles after having burned the leaf piles. (but as I said, you might not actually have to burn the piles -- just simply throw away the leaves, if that is possible. If not, then just keep setting fire to them until they finally turn to ash. (again, usually about 4 to 5 tries). Then do the trash can head trick to pick up the ash piles.
When you're ALL done, hit CTRL + SHIFT + C again, and type testingcheats off. Close the window by hitting CTRL + SHIFT + C again.
https://drive.google.com/open?id=13gOIpH4JmOWdk_bpgX-oS1EHP6f9Agnr
This is not a permanent solution; the Sims team will have to work on that. But for now, the above process DOES work... so you can still enjoy Seasons in your game!
Happy Simming!