This is a very subtle bug probably related to a previous one fixed by Tim Lindner. The bug is found with coco2 emulation but not coco2b, coco3, or coco3h.
Anything saved to a .dsk file has the last byte of a sector trashed. Strangely enough, the same bytes can be read correctly. I have tested this by saving Basic programs as both binary and ascii files and examining the results with a Windows raw file editor.