Not necessarily... space can be omitted when you use an apostrophe.
That doesn't match the examples in my Τριανταφυλλίδης, but that's admittedly 20 years old.
It mentions replacing letters by apostrophes, but doesn't say anything about removing spaces (in the places I looked), and the examples for the rules on apostrophes all have separate words, whether it's the last sound that's removed (σ' εσένα) or the first one (θα 'χει).
Have spelling rules changed in this respect?
The problem might have been created in the "digital era" as previously I don't think that they ever required to discuss this. Adding a space between the words wasn't even an issue for ancient Greek (same as accents on the words). I've never got corrected at school when I was writing μ'αρέσει, σ'εσένα, θα'ρθει, etc. I searched online but I can only find some discussions on forums. No actual book will include this info but if you find one, please share!