It may be related, but it looks like a different bug. At any rate, trying your workaround and doing just the resize, with input and output formats (.pdf) the same:
on jpegFromPath:imagePath toPath:newPath newWidth:theWidth newHeight:theHeight compressFactor:compFactor
set theImage to current application's NSImage's alloc()'s initWithContentsOfFile:imagePath -- load the file as an NSImage
set newImage to current application's NSImage's alloc()'s initWithSize:(current application's NSMakeSize(theWidth, theHeight)) -- make new blank image of target size
-- draw from original to new
newImage's lockFocus()
theImage's drawInRect:{origin:{x:0, y:0}, |size|:{width:theWidth, height:theHeight}} fromRect:(current application's NSZeroRect) operation:(current application's NSCompositeSourceOver) fraction:1.0
newImage's unlockFocus()
set theData to newImage's TIFFRepresentation() -- get bitmap as data
set newRep to current application's NSBitmapImageRep's imageRepWithData:theData -- make bitmap from data
set theData to (newRep's representationUsingType:(current application's NSJPEGFileType) |properties|:{NSImageCompressionFactor:compFactor, NSImageProgressive:false}) -- turn the bitmap into JPEG data
(theData's writeToFile:newPath atomically:true) -- write it to disk
end jpegFromPath:toPath:newWidth:newHeight:compressFactor:
The catch is that it might be a bit harder to use from FileMaker -- it may need to be called by an external script.
Meanwhile it would probably be worth logging another bug on sips.