I have only recently started using Topaz Sharpen and Denoise AI. Every now and then, when I apply the auto mode, which then should put the new version back in LR, I get a message on the LR screen with the "old" version of the image, and the message "The file uses an unsupported colour mode". When I click on that in the film strip, and then on the blank Tif square in the strip, the applied result pops up. Thanks for your assistance.
I just spent a lot of time troubleshooting the same issue. Long story short I went back to the original .DNG file and tried the same thing in Gigapixel as I was trying on the edited .tif and got an error that I was exceeding the maximum file size with Gigapixel. I went back to the .tif file I was editing and switched the Gigapixel from 4x to 2x and it imported without issue so it's possible that the "unsupported color mode" is a red herring and it's actually exceeding the maximum file size of LR. I emailed support to confirm but that's what worked for me.