I had this issue for a couple of days, and got tired of waiting for a WhatsApp software update.I got tired of waiting for a fix, so I performed the below steps to resolve the issue .I was restoring to an iPhome 7 Plus, however, this should work for any iPhone model.
1- make a local iTunes backup
2- delete WhatsApp application and then download again from app store
3-launch WhatsApp, then wait for it to restore. It should get stuck at "finishing up 99%"
4- Press home > settings > iCloud> storage> manage storage > documents & data> WhatsApp> edit > delete all
5- press home button, launch WhatsApp again, it should still be stuck
6-press to return to springboard/home page
7- double click home button to launch all open apps, swipe up to force close WhatsApp
8- launch WhatsApp once more, this time it will say optimizing. It will then go to a page where it says trying to restore from iCloud. It won't be able to, since we deleted it prior. On the top left select "skip".
9- it will then go to the next screen with all your groups at the top, and individual conversations at the bottom.
10- to create a new WhatsApp iCloud back up, go to settings in WhatsApp app> chat>chat back up>backup now
Thumbs up if the steps above helped you out. I take no responsibility if your messages get deleted, these are just the steps I took to resolve restore issues on WhatsApp.
Let me know if you need any clarification on any of the steps.