Amusing oddities with SpriteExtend and JPEG

I submitted this as a ticket on the bug tracker, but because nobody reads that any more, I’m also posting it here.

Here’s the program (hopefully properly formatted this time!) and the JPEG can be found in the ticket.

spr_output%=FALSE
ON ERROR PROCerror
in$="Landscape_0175/jpg" 
out$="JPEG" 
SYS "JPEG_FileInfo",1,in$ TO ,,width%,height%
DIM s% 6000000
!s%=6000000:s%!4=0:s%!8=16:s%!12=16
REM Try setting R3 to 0 or 1
SYS "OS_SpriteOp",15+256,s%,"jpeg",1,width%,height%,(4<<27)+(90<<14)+(90<<1)+1
SYS "OS_SpriteOp",60+256,s%,"jpeg" TO r0%,r1%,r2%,r3%:spr_output%=TRUE
REM Try setting R4 to 0, 1 or 3
SYS "JPEG_PlotFileScaled",in$,0,0,0,3
SYS "OS_SpriteOp",r0%,r1%,r2%,r3%:spr_output%=FALSE
SYS "OS_SpriteOp",12+256,s%,out$
END
DEF PROCerror
IF spr_output% SYS "OS_SpriteOp",r0%,r1%,r2%,r3%:spr_output%=FALSE
REPORT:PRINT " at line ";ERL:END
ENDPROC