why would you put a begonia in a gritty mix?

shouldn't begonias be in a more moist soil? You don't expect begonias to grow in sand. just like you wouldn't put an African Violet in sand. Some plants take fine to grit - and others really do need dirt.

Well, the gritty mix is not sand. I wouldn't put any plant into sand. The gritty mix can hold a lot of moisture.

I should also mention that last year I put two tiny begonia starts in the gritty mix and they are fine. My problem now is with transplanting established begonias.

IME with Begonias, they can have harder adjustments than a lot of other plants to disturbance, but worth it soon after. The tips might falter, but there's usually 2-3 new stems that sprout lower down to take place of the one lost. Did you do any trimming of the roots?

I wonder if your mix was very dry and just needed a better initial drink? If you did this outside to a plant that had not been out yet, it's possible the leaves are burned. Begonia leaves can just kind of melt if they are burned, with seemingly no brown crispy phase at all sometimes. Same thing with frost bite, or just a wild sudden variance in temp. Just about anything can mess up those with really delicate leaves, but it's usually just the leaves. One of those plants that can drop/re-grow leaves quite easily.

Can you show a pic or two?

I'll try to get some pictures tomorrow purple. I am actually a lot less concerned about the two bigger ones. They seem a bit less droopy. The smaller ones on the other hand...:(