Just updated from 2017 SP5 to 2018 SP5. After the update I cannot drag to move any components in an assembly after 1 mate has been added. Once a component(part or subassembly) is inserted, it will drag no problem, as soon as I add any type of mate, drag to move no longer works. This will be any mate type, even a single parallel mate will lock the component. This is not a 100% case, it seems to be random as to which parts it affects. I can add the same component twice to an assembly and have one lock up and the other is fine. The issue is not confined to Large assembly mode. Not sure if this is crucial, but I've been doing testing with only adding parallel mates and sometimes I get the "constrained" symbol next to parallel mates, sometimes not(see below).  Presumably this is some sort of issue related to upgrading to 2018 as I've been using 2017 for years with no issue. Perhaps a Windows setting? Performance setting?

The following are true:

• No components involved in the mate are "fixed"
• I am not trying move components inside a sub assembly. 
• I have the "Move components by dragging" boxed checked under "Options->Assemblies". 
• "Move with triad" does not move the component
• The "Move Component" command does not move the component
• The "Rotate Component" command does surprisingly rotate the otherwise locked component