8. Some objects have disappeared from my view. I could find them by pressing Ctrl+Space sequentially and noticed that the opacity for those objects was 0, so, I moved the opacity slider to the right, and the objects were visible again. I saved the file, but when loading it again, the same objects have disappeared again (and again the opacity went to zero). Even without closing the file, sometimes the program changes the opacity of such objects to 0 again, for example after selecting one view of the list. How can I fix this?
This behavior happens when you change properties of an object in a keyframe without updating the keyframe. If you want to alter the appearance of an element in a keyframe, you need to press the Update button to saves the new state in the selected keyframe. For more information, please watch https://youtu.be/k2Q0-UY0ixU.