When I choose export, I choose "LRT Pro (TIFF, 8bit)" and Lightroom still exports the files as JPGs. I can also choose the 16bit version for TIFFs and it still exports to JPGs. I tried updating Lightroom tonight to see if it would help, but nope, same thing. I'm using Lightroom Classic 9.0 and LRTimelapse 4.8.3.

I know it used to export to Tiffs as I have some folders with the Tiff output still, but it looks like it has been exporting to JPGs recently, even though I've been picking TIFF.

Any ideas what could be going wrong?

Apart from LRTimelapse 4.x not being supported anymore, there is a bug in Lightroom 9 that causes this issue.
Please see the thread in the LRTimelapse 5 area: https://forum.lrtimelapse.com/Thread-tif...ightroom-9
I've also informed about this via newsletter. You might want to subscribe to hear, when it's fixed: https://lrtimelapse.com/newsletter
I'm closing this thread here, we'll discuss everything about this in the other thread.