Best way I have found is to create a part with no geometry in it, add the relavant part information (in this case your gasket description, part number, etc.), and then insert this "blank" part into the assembly. This way you do not have to model the part as you stated and you can keep your standard bom without edits. I generally try not to modify the actual bom tables on the drawing since they do not automatically update well in future revisions when additional parts or sub-assemblies are added. Hope that helps.
I have just seen the same thing. I already had a part modeled but we add our own 'description' column. I am able to type in there like normal but when I click out of edit the text goes away. We have been doing our BOM's like this for years and this is the first I have seen.
Side note: I also noticed I am unable to 'exclude from BOM'. I choose to exclude it in my BOM so I thought it just wasn't taking but when I go back to my assembly and try to exclude it there it says it is already excluded but still shows up in my BOM. I managed to fix that by changing the configuration it was attached to but no luck with the disappearing text.
Right click the model that the BOM is linked to, and 'Load model'. After this, the table will be editable again.