It is almost certain that the corruption is caused by the SD card, either because it is faulty, because the file system is corrupted, or because the device does not know how to use such a large card. This position is supported by KDE Connect also producing corrupted files.
You can easily test this by changing CC's book directory to the device's main memory, sending some books, and seeing if they are corrupted. Another test would be to try a different SD card that is smaller and made by a known reliable company. If your device is rooted then a third check would be to do a file system check of the SD card as
described on this page.
If for some reason you don't want to do these tests and if you have purchased CC then contact us for a refund.