It sounds like too much sun to me, leaf scorch. I'm not a Sans expert, but I'm not sure that they like full sun, I've read they prefer bright light (indirect sun, but in a "bright" location).

Planto

I agree. There are tropical sans and desert sans. I think the direct sun is too much. Try morning sun or just bright lighted spot in the garden. Try not to cut too much off, They could heal themselves. Still have the spot to see.