My Actea's ahve the same symptoms and as they likethe same conditions as astilbe's, it's probably scorch from the weird weather we've had. I think I contributed to mine by watering while it was still v hot & sunny, but it was the only time I could do it for a few days so took the risk.
Try a bit of TLC & watch for new leaves.
If nothing improves with the more normal weather, check for spider mite infestation or similar. Some of my less well established plans are succumbing as they have not been robust enough to cope with conditions and can't fight the little blighters off.