I am using Topaz AI as a Plug In Extra within LRC. My preferred output from Topaz is DNG.
I will often select multiple pictures to be processed at a time and sometimes Topaz 'chooses to output these back to LRC as TIFF files. Maybe happens one in ten times, always when processing multiple images.
What file types are you working with as your original files in Lightroom?
If you are working with RAW files, it is recommended that you use the File > Plug-in Extras > Process with Topaz Photo plugin , which will generate a DNG file for them.
The Photo > Edit in > Topaz Photo AI plugin will give you the option to change the file format for the edited image; however, this will not include the option for DNG.