It looks like there is either a bug, or incomplete data with some of the compendium spells, or just a part of the D&D 5E by Roll20 character sheet that isn't exposed to players. I just tried this with Charm Person , which wasn't showing the 'SPELL SAVE DC' on a roll, while Harm was working as you showed. I added Charm Person to a character and confirmed that it wasn't showing the SPELL SAVE DC. Then I opened up Charm Person and changed the Spellcasting Output from SpellCard to Attack. Then more options were displayed, including Saving Throw. I changed the Saving Throw from 'None' to 'CHA', changed the Output back to SpellCard, then rolled again and now the SPELL SAVE DC displayed. I went back and changed the Saving Throw back to 'None' again, and the SPELL SAVE DC still displays. So it appears that manually toggling the Saving Throw portion of a spell toggles the @{spellconcentration} flag on if it's not previously on before. I don't know if there's a way to toggle that back off afterwards though. At least not that I can find on the sheet. It looks like that's something the sheet worker is doing in the backend and just isn't displayed to players. It also appears that this is not a global setting that you can set for all spells at once... you'll have to manually adjust each spell on each caster. I'm not sure who/where to give feedback to about this to have it exposed.
