We will add a workaround for this, but the bug looks like it is on the TortoiseSVN side. (CORRECTION: It's not a bug on the TSVN side, but part of the API standard.)
Its drag & drop data object is giving out streams which are initially positioned at the end of the data. Most likely the data was written into the stream by the drop source (TSVN) and then the position was not reset to the start before the source handed the stream to the drop target (Opus).
I have reported this to the TSVN devs to see what they think. It's possible there is something in the drag & drop spec which we are not aware of, but I suspect this is something that has been overlooked on their side.
Either way, the issue should no longer affect Opus after the next update (12.5), as we've added code to prevent it.