I might have found a serious issue with the tool.
When loading anything from Hollow Fragment (PC), it will open the mesh preview the first time. After that, preview fails to load further for either Hollow Fragment or Hollow Realization. Last time, I searched the registry for any key related to the tool and removed it, and it suddenly worked again.
I have actually narrowed down which registry key is causing this issue, though why it causes it, I am unsure. I just know deleting it and relaunching the application results in the preview working again. I also don't think the issue is limited to HF, as it is registry specific.
Registry key causing the issue is actually a few keys:
Computer\HKEY_CURRENT_USER\Software\ \
Computer\HKEY_CURRENT_USER\Software\ \SAOViewer\
Computer\HKEY_CURRENT_USER\Software\ \SAOViewer\1.0.0.0\
My theory is that it is because of the blank space in the structure name, but it very well could be that the entire keyset itself is causing the issue.
In the meantime, I can simply delete this key manually because I use the tool sparingly as I progress in my project, but it admittedly becomes quite inconvenient.
Anyhow, if anyone else here has this issue, just open regedit (windows key + r, type in "regedit" without the quotes, and find and delete the key with the name "SAOViewer" without quotes. This will temporarily fix the issue until you close the application again.