Why did my fill turn to stroke after copy and pasting the shape in Indesign 18.2.1?

Are you using object styles or did you modify the Basic Graphic Frame object style? If you have an object style applied to something you copied and there is a style with the same name in the target document the target document’s style will be applied.

Say you have the style Figure in your Document A which uses a grey fill and no stroke. In Document B there is also a style Figure which has a while fill and a black stroke. Copy something using the Figure style from Document A and paste it into Document B. It wil have a white fill and a black stoke. When pasted items don't look like you expect the first thing to check for is inconsistent styles.