MacOS: 10.15.6
Affinity Designer: 1.8.4
I've noticed there are a few bug reports around symbols and I'm not sure if this is related or new but hopefully it'll help debug. I've reproduced a minimal example.
I've created two artboards and added a line using the Pixel Persona and a rect using Designer Persona to artboard 1 and made them both symbols. I then added a copy of the symbol to artboard 2. Modifying properties of the rect work as expected.
Modifying properties of the line seem do not update as expected.
Rotating the line seems to complete break and forces the line to disappear from artboard 2. The preview image however does show that the line has been rotated.
I'm not sure if this is supposed to work this way or not with symbols, but when the (Symbol) layer is selected (not the Rectangle or Pixel layer) and the object is rotated, it only rotates the instance of the selected element and none of the other symbols are adjusted.
Thanks for the support, this one is causing some headaches in a project I'm revisiting.