I imported several .cr2 images into Lightroom, exported them as .tif without any adjustments...
There's no such thing. If you are seeing an image on your screen and/or exporting a TIFF, the raw data is being interpreted in one of a countless number of possible legitimate interpretations. If you "made no adjustments" it just means you are leaving the default settings Lightroom used to initially open the raw file. What those default settings are depends upon what they have been set to, either by the software designer or someone who has changed them after installation onto the computer you are using.
When you export as TIFF you "bake in" many of the choices that were made regarding how to interpret the raw image data. White point, black point, white balance, etc. are all much less flexible with a TIFF file than with a raw file. If you then try to move any of those things very far using the TIFF, you'll get artifacts. Sure, a TIFF from a 12 or 14 bit raw file can be 16-bit, but that does not mean it contains all of the information the raw file contained. Raw files contain a single monochromatic luminance value for each photosite (a/k/a pixel well or sensel) on the sensor. TIFF files (as you are using them) contain RGB values for each pixel in an image. These are two very different things.
One thing to keep in mind is that Lightroom does not use demosaicing algorithms produced by Canon when dealing with .cr2 files. Instead, Adobe develops their own algorithms, primarily through what we might call reverse engineering.
It was shot on an 80D using a 17-70mm Sigma Lens in the mRAW format.
mRAW is not really a true raw image format that preserves the monochromatic luminance value collected by each sensel on the camera's sensor. Raw files directly out of the camera have only 14 bits per pixel. The pixels are all monochrome. In sRaw and mRaw files, the images are already demosaiced, and there is more color information per pixel. It's not 14*3 = 42 bits per pixel, though. The data are converted from RGB into a sort of linear YCbCr format with 1:2 chromaticity subsampling.
That's probably the primary source of the chrominance noise that seems to be a major contributor to your artifact.
The artifacting does not occur when observing the .cr2 file in a viewer like IrfanView when increasing the contrast there.
Then the issue is most likely with Lightroom's demosaicing algorithms for Canon mRAW files. Irfanview is probably displaying the JPEG preview image embedded in the mRAW file. The JPEG preview is generated in-camera using Canon's demosaicing algorithms. (Presumably the same algorithms are used to create the RGB values for each pixel of the mRAW files). You will get results nearly identical to the in-camera generated JPEG preview using Canon's Digital Photo Professional 4 software if DPP 4 is set to the same settings as the camera's settings at the time the JPEG preview was generated when the image was shot.