Keywords no longer synced from LRC to LR

I've just verified that the one way, one time transfer of keywords and location data from LrC to Lr still works. Likely there's a step missing in your workflow, but to reiterate the key elements of the procedure are as follows:

1. Ensure that a Smart Preview has not yet been created in LrC. If you have already created one, e.g. during import, you must delete it in order to facilitate the subsequent keyword transfer. Do this for all images you intend to keyword and sync.
2. Once you have finished keywording, and any existing Smart Preview has been deleted, you must then save metadata to XMP (if you don't have that automatically enabled via the Catalog Settings, you can manually force that write by selecting the target images and do Ctrl+S). This is a key element to facilitate the metadata transfer.
3. Once that is done, sync the images to the cloud, either by adding the images to a synced collection (best), or by adding them to the All Synced Photographs special collection (in the LrC Catalog Panel). No need to keep enabling/disabling syncing in LrC, as the latter allows selective syncing of images.

The point about deleting any existing smart previews is that when syncing an image to the cloud from LrC only smart previews are uploaded, and if LrC knows that a smart preview already exists it copies that existing SP rather than build a new one for syncing purposes...and if that SP was created BEFORE you save the metadata to XMP then that metadata will not be in that SP copy when it syncs. So you have to ensure you get the sequence correct, specifically.....ensure no existing SP, then keyword, then save to XMP, then sync and the specially created new SP for syncing will be created from the image file and will thus include the metadata stored in the image's XMP.