Using a gallery inside a form card (or inside a card within the scrollable canvas) is not (nor has it ever) been supported. If you select a card, and try to insert a gallery on it (via the Insert menu), then it will not work (it will be placed outside of the form/canvas). This is a scenario that is not tested, and as such we cannot guarantee that it will continue working in the future.
The fact that you can bypass that limitation by copy/pasting a gallery inside a card is a bug in the product. It happens that this bug allows many people to do things that the product team never really intended to, but as I mentioned above, it may break in the future (although we'll try not to).
If you really believe this should be a supported feature, please consider creating a new entry in the PowerApps Ideas board, so that this will start being officially supported.