I just installed SW2018 last week. I've not had this issue before so I think this may have something to do with the update.
When I insert a design table into a part, after creating the table, it does not show up in my feature tree. I also miss the option to always show design tables in Tools > Options > System Options > FeatureManager. For all other items the dropdown menu has 3 options: Automatic, Hide, and Show; but for Design Table the 'show' option is missing. When I have it set it to automatic the table does not show up in my feature tree.
Because of this I can now not edit my design table after creating it. It does seem that the design table was inserted into my file, because the option to add it becomes greyed out under Insert > Tables > Design Table. This same issue happens both in new files and in files that were created in SW2017. In files that were created in SW2017 that already had a design table in them it also does not show up.