Yes, syndicated copies should canonical back to your original, or at the very least link back to it, so the ranking signal consolidates to your version rather than fragmenting across copies. That is the correct practice and what protects your original from being outranked by the very content you provided. The harder half of the answer is that partners often will not agree to it, because a canonical pointing to you cedes the ranking value to your site and they want that value for themselves. So the realistic move is to negotiate canonical or attribution up front and accept the risk that comes with not getting it.
The reason a canonical is right is consolidation. A rel=canonical on the syndicated copy tells search engines your version is the authoritative source, so the ranking weight the content earns flows back to your original instead of being split or handed to the partner. Without it, two competing copies leave Google to pick a winner, and on a stronger partner domain that winner is often not you. The canonical is the clean mechanism for getting reach while keeping the ranking, which is why it is the practice to push for.
The honest reality is that compliance is not guaranteed and frequently withheld. From the partner’s side, hosting your content and then canonicalizing it to your site means they do the publishing work and you keep the SEO benefit, so many decline outright or offer only a footer link instead. A request does not ensure it happens, and assuming partners will comply sets you up to discover too late that no canonical was placed. Where you cannot get a canonical, a prominent attribution link back is the fallback, weaker but better than nothing, and you go in knowing the copy could outrank you.
Settle this before you agree to syndicate, not after. Make a canonical to your original, or at minimum a clear attribution link, an explicit term of the arrangement, and confirm it is actually in place once the copy goes live. If a partner refuses both, weigh the reach against the real possibility that their copy outranks yours, and decide with that risk in plain view rather than discovering it later.