To start out, no this isn't a duplicate of Export unfinished image render. This is a question about why the answer to that question stopped working in blender 2.8.
Last night I stopped a render half way through (bottom to top render mode) and saved the partial image just as normal. Then this morning I started the render again in top to bottom render mode. When the render progressed to where is was last night I stopped it, and the render disappeared costing five hours of render time. I tried fiddling with the cache setting and all sorts of stuff. I even tried using render layers but the partial render image still keeps disappearing if the render job is canceled.
Here are my output settings in case this helps.