There are a few class features in the game that require concentration, but are not spells: A Trickery Cleric's Invoke Duplicity, a Glamour Bard's Mantle of Command, a Graviturgist Wizard's Adjust Density, etc. They are all phrased in a similar way concerning concentration:
for 1 minute, or until you lose your concentration (as if you were concentrating on a spell).
Going into a rage stops you from keeping concentration on a spell:
If you are able to cast spells, you can't cast them or concentrate on them while raging.
But, if a multiclassed character (with one of the above subclasses, plus barbarian) uses such a concentration-based feature then goes into a rage, are they able to maintain concentration on that feature?
For example: Timmy the Trickery Cleric/Barbarian uses Invoke Duplicity, then enters a rage. Is Timmy able to continue concentrating on Invoke Duplicity?