The free tools for HEIC conversion on Windows are lame. Don't convert to jpeg with other software. Instead, open in Lightroom CC and click the image, selecting from the drop down, "Edit in Photoshop". The adjusted HEIC image will open in Photoshop as a 16 bit tiff with adjustments intact, which is a big clue that the bit depth in HEIC taken with an iPad or other mobile iDevice has more that 8 bit per depth quality, my guess is ten bit depth per channel.