For some reason my Houdini keeps defaulting to thumbnail preview.

Any ideas, or does it use Windows preferences?

Yes, this drives me crazy as well. In a folder with several 8k images this always means time for a coffee break.

+1 for me, it also drives me bonkers. If i knew a way to turn it off i would. Best mark

I have been same issue,But Some Freind really help me,If you have Multi Storage Or many texture include in different path,You get some problems Also when your Render image ,Put All texture or your element only folder currently using in your scene include in Folder temp project,After When you finish your project ,Archive As zip or rar file…Some people get problems to navigate several time to folder or problems to render,Because multi textures include of 1TB or more And different path.So Before Start your project find Texture ,Models,Element,etc…And put them In your temp Folder.
If You Notice Before render Process Ram Take space read All Element or Texture to compiling process,That take Some Time.And then start To render.

Oh, I thought it was something I switched on and forgot about it. If ather people are having the same issue, it might be a bug.
I will submit to Sidefx and see what they say.