Are favorites in the Photos app suppose to sync between devices? I use Apple Photos on my Macs and on my iOS devices. I can flag photos via the Heart icon as favorites, but in my case they are not synced between devices.

Is this a bug, or is this meant to be like this?

ps: I saw the question Are "Favorites" and edits synced with Photos for Mac/iOS?, but it neither attracted answers nor is it it particular current - that's why I ask this here and now.
They will (or should) sync between devices if you are using iCloud Photo Library, otherwise, e.g. when syncing just the PhotoStream, favourites do not sync.

I just tested this on a device running iOS 12.1.3 and macOS Mojave 10.14.3 by favoriting a photo in the iOS photos app, and not only did the photo appear in the Favorites album in Photos.app on Mac, the Favorites album was automatically created in both places. (I don't use Favorites, but they do seem to work for me.)

According to Apple's Support article for iCloud Photo Library, edits do indeed sync between devices. I also confirmed this just now by cropping a photo on my Mac, and watching the cropped photo appear on my iOS device.

NB: if you are trying to sync favorites between devices using a hardline sync such as iTunes, I am unable to describe what the correct behavior for that is, as I don't have a setup to test it with currently, nor could I find any information on Apple's support website.