Hi Patrick,
Before the recent update (4.8.4) there was a known issue with index table and custom field search.
Let’s try the following steps:
1. Make sure that the Remove dash like characters? option is turned OFF on the index table options page: http://i.imgur.com/GF2iibS.png
2. If the _sku custom field is selected on the index table page, save the options once again, so everything is refreshed.
3. Delete the current index and create a new one: http://i.imgur.com/9IHjvTN.png
4. Make sure that the index table engine is selected on the search instance options page.
Once the new index is ready, try searching again for the SKUs again. If the problem was related to the bug with the previous version, it should start working.