Preferences > treat jpeg next to raw option not working

As already explained, this setting controls subsequent imports only.

For images already in your Catalog, it is an easy matter to temporarily filter (whether a folder view, or a collection view) - e.g., hiding all JPGs.

If you really do not want the JPG separate versions there at all, you can get rid of them. Being careful of course, about any camera JPGs present which lack a Raw alternative! 

For cameras which offer the option, I do prefer to decisively shoot to  either Raw or camera JPG in the first place. There is IMO a lot to be said for the clarity of that. Incidentally, for the rare times I do want a camera-processed JPG version of an image that was shot to Raw - by doing this via the camera menu afterwards, a fresh image number gets applied, in my experience - and thus, no automatic Raw+JPG pairing then happens.

Sounds like you are looking at images already in the catalog. This preference setting is for newly imported images only. Synchronising the folder after you've changed this setting works if you change it from not treating them separately to treating them separately (the jpegs will be imported), but what I understand is that it does not work the other way round (anymore, it used to work too).

Please take a look here:

Raw and JPG files | Lightroom Queen Forums

"treat jpeg files next to raw files as separate ph... - Adobe Support Community - 10178082

JPG and RAW display together in Library - Adobe Support Community - 11308599