Excluded data because it cannot be saved

Hi. One of our designers in 3DX 2018x has issues with this error saying that "Excluded data beucase it cannot be saved".

He is working in an assembly and this contains a sub-assembly with screws that are in status Released.
There has NOT been any changes to this sub-assembly, but still 3DX is complaining that this has to be excluded in order to save the main assembly.

This kind of error should not appear and the designer has to remove the released sub-assembly and add it to the main assembly again in order to save. This takes a lot of time.

Does someone know how to fix this or knows about a workaround?

Thanks!