This is more by way of a heads-up than anything else.
We have noticed that, if we override the mass of a part, the 'Drawing Mass' Custom Property we derive using an equation fails - it becomes zero, because the equation has an error. We use this Custom Property to report the mass of parts in drawings and in PDM Standard data cards. We are using SW2018, SP5.
We contacted the VAR and asked about this, and they pinpointed the problem to the internal property 'SW_MASS', which seems to be deleted whenever the mass is overridden.
They have reported it as a bug to Solidworks, and a SPR has been raised (they didn't tell me the SPR number, and it is unfeasibly difficult to find a given SPR in the register, as we know). It is unlikely to be fixed in SW2018, as SP5 has been released, but the VAR and I have separately verified that it has been fixed in SW2019, SP2.
So, if you use a Custom Property based on SW_MASS and you see strange behaviour in SW2018, don't despair - all will be OK when you move to SW2019!