Alfie from the compost forum -

You need more compost on your Mutant lemon-rose scented geranium. Doesn't compost fix just about everything?

Deb
who is seeing just what's up on the Geranium Forum

Are rose geraniums hybrids?

Could that one branch be mutating back to type?

If I were you, I'd prume that branch right now (and compost it :).
Then I'd (go Deb one better by) watering in, & foliar-feeding with, compost tea.

- Evelyn, know-it-all from the Compost Forum

It could be a sport - try cutting and rooting it if you like it.

One sported on a rober's lemon rose here too. Maybe it's a sporty type.

The bulk grower for local greenhouses, has variatons of varieties, that aren't as good as the ones at the top suppliers. It may not be a best example of a rober's lemon rose.
It rooted easily, stuck back into mama's pot.