On almost every torrent, and after doing a "Check" on a completed download, the following error message is displayed under "Status":
error seeking: An attempt was made to move the file pointer before the beginning of the file (131)
The version prior to v3.12 was also doing the same thing, just not as often.
Lastly, v3.12 simply will NOT perform an automatic end-of-download check or directly MOVE the downloaded files. It simply sits FOREVER on the "Checking" status after appearing to have completed the check.
You seriously need an "Download prior versions" section on this web site so we can back up to an earlier fully working version until things like this get resolved.