I have ran into an issue with 2d symbols and the multi view plane shortcut (m). When editing a 2d symbol if you switch into multi view and back (in my case accidentally hitting the M key), when you return back to your design layer all instances of the symbol in the design layers disappear. This problem appears only with 2d only symbols, not 3d or hybrid. I have found the way to get them back is to either; in design layer enable and disable multi view planes (this works most of the time). Or you can add a 3d locus to the symbol via the resource manager creating a hybrid symbol. Both options causes them all to reappear in the correct spot. I am running V2019 SP1 Build 460795 on Windows 7.