I have seen this a few time and so has my co-workers. We have the main assembly then a carcass assembly with the panel parts inside it. In the main assembly if you pick one of the panels and try to make transparent it will not make it transparent. It will only make the whole assembly transparent if you select the assembly. The assembly file is to large to add here.
Now after making the side panel transparent.
Now after I make the assembly transparent.
This is in SW2018 SP5.