Okay so for those who may find it useful, I figured out the issue.
The fix is that, at least while compositing and rendering the chroma-keyed clip, disable optimized media. At the global level that can be done by going to
Playback > Use Optimized Media if Available and ensuring it is unchecked.

For me, this meant that the Fusion tab now matched the rendered export, where the key was not keying out the desired color completely. You can then adjust the key to cover missed regions, then export, and finally get the desired region entirely keyed out as expected.
I'm not familiar with the internal workings of Resolve, but I'm guessing the cause of this issue has to do with the color range of the compressed optimized footage being smaller and thus easier to key than the uncompressed raw footage. So, when applying a chroma key to optimized footage in Fusion, the key becomes calibrated to the "easier-to-key" optimized footage. Then, when rendered in export, the uncompressed raw footage is used. This is sorta "harder-to-key" due to its increased color range, and so the key calibrated to the compressed footage may drop out occasionally on the raw footage, which is exactly what I was seeing in the render.
Hopefully this helps someone down the line! I'm happy to have found the issue and glad it was fixable!