Index rebuild not working with SQL Server 2016
As mentioned in the comments there are various factors that could cause fragmentation, and following are the areas that i can think of in your case.

Fill Factor
Seems it's already lower than normal on particular index. However, consider fine tuning with it (in development server) if following options are not applicable in your scenario.

Auto Shrink
Verify by any chance the AUTO SHRINK option set to ON at database level as this could cause heavy fragmentation 

GUID Columns
There is also possibility when the entries are inserted without an order, switching to SEQUENTIAL-GUID would be better option (if possible)