Exported TIFF images are actually JPEGs

I have the following plugin code:

local Plugin = {
    hideSections = { "exportLocation", "fileNaming", "metadata", "video" },
    allowFileFormats = { "TIFF" },
    allowColorSpaces = nil,
    exportPresetFields = {}
}
function Plugin.startDialog( propertyTable )
    propertyTable.LR_format = "TIFF"
    propertyTable.LR_tiff_compressionMethod = "compressionMethod_None"
end
function Plugin.processRenderedPhotos( functionContext, exportContext )
    local exportSession = exportContext.exportSession
    local nPhotos = exportSession:countRenditions()

    local failures = {}
    for _, rendition in exportContext:renditions{ stopIfCanceled = true } do
        local success, pathOrMessage = rendition:waitForRender()
    end
end

When I run that code on OSX, the exported file will be a JPEG file, instead of, as requested, a TIFF. What is going on here? At the very least, I would expect some sort of error output, but Lightroom just happily produces a JPEG file.

On Windows, the code works as expected and always produces a TIFF.