LIGHTROOM

Yes, this sounds like you are in a 'No edits' smart collection. As soon as you make an edit, the image no longer fits the criteria of the smart collection and so it disappears from it. Sorting does not make an image disappear from loupe view, so I don't think that can be an explanation.

Are you talking about Lightroom (Lr icon) or Lightroom Classic (LrC icon)? Please also tell us the version NUMBER of your software (we need the version NUMBER and not words like 'latest' or 'up-to-date').

We really need a lot more detail about what you are doing; however as a wild guess, it could be that you are in a smart collection in Lightroom Classic, and one of the conditions of the smart collection is that the photo has no edits, so the photo disappears from the smart collection but it is still in your catalog, it has not been deleted. Another wild guess is that your sort order is set to Edit Time and so when you edit the photo, it moves to the beginning or end of the sequence of photos.

But enough wild guesses, explain what you have done in detail, step-by-step, starting from the beginning.