After providing a diagnostic report through my switch itself and via the Homey app, Rithum support has confirmed this is an issue with the latest Homey firmware, so they are now investigating @Doekse
They were able to replicate the issue on their side. Existing flow cards work fine, but new flow cards do not.
If you own a Rithum switch and use it with Homey, do not delete your current scenes until this fix is implemented or you won’t be able to get them back.