When I make an edit on a photo in a collection, LR makes the exact same edit to the photo in it's original folder as though they are synced. Realised this has only started happening since I updated LR a few months ago. How do I stop this?
This has always been the case: a given photo within Lightroom Classic always appears in a Folder view that corresponds to where its source file is recorded to be located on disk. This photo may - or may not - also appear in one or more Collections (because it has been added to those) and also in one or more Smart Collections (because it qualifies for their "rules" of membership).
But this is all the same single image. Latest edits are seen in all the places where it is seen.
Collection membership is a little like being a member of this forum: it is the same "you" here, as the person who is also member in (say) another forum, or various clubs or societies, but there is always some real physical location where you are to be found too. Or if you went physically missing from that location, you would still have a "last known address" in the records - that is how the Catalog works.
You have the option of making a Virtual Copy if you want to edit the appearance of two discrete image versions separately. You can do that at the same time as adding an image to a Collection (even as part of making a new Collection, you can check an option to include current selected images, AND you can check an option to have these added as new Virtual Copies). Otherwise you can make a VC first (or a Proofing Copy, which is equivalent) and then add that to the desired Collection. So in this Collection you may only see that VC and not the master copy of that photo. And a different Collection could hold another, independently editable, VC also deriving from this same photo that you imported.
However all these versions are necessarily presented in the Folder - unless you stack or filter some of them away from view - since for all these virtual copies and for the master version, this folder is where the physical file genuinely lives, that underlies them all.