There are too many unknowns to even guess what could be causing your issues. It could be the PNGs, bugs with bitmap support in general which is quite limited in Animate, bugs with Layer Parenting, of which there are many, a combination of these, or something else. As a general advice I'd suggest you leave the PNGs for later and start exploring Animate and symbols by creating simple vector artwork within Animate. Once you get to a point that you know what you're doing and how things behave with simple vectors, you will be able to trouble-shoot and introduce bitmaps in the workflow.
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