Quick update:
I think I've narrowed it down to where the error is occurring. When attempting to load the Stage textures from the battle directory, it's looking for different filenames in Hotfix 10, for whatever reason. Fighting the exact same enemy in the same location, it goes from:
[00000827] TRACE: open_file: STAGE05.D (LGP:battle)
[00000827] TRACE: open_file: STAGE05.B (LGP:battle)
[00000827] TRACE: open_file: STAGE05.P00 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.T00 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.T01 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.T02 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.T03 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.T04 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.T05 (LGP:battle)
[00000827] TRACE: PERSONAL TEXTURE - D:\Program Files (x86)\Square Soft\Final Fantasy VII\The_Reunion/CUSTOM/upscale/Texture/battle/
STAGE05_T00_00.png[00000827] TRACE: open_file: STAGE05.P01 (LGP:battle)
[00000827] TRACE: PERSONAL TEXTURE - D:\Program Files (x86)\Square Soft\Final Fantasy VII\The_Reunion/CUSTOM/upscale/Texture/battle/
STAGE05_T01_00.png[00000827] TRACE: open_file: STAGE05.P02 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.P03 (LGP:battle)
[00000827] TRACE: PERSONAL TEXTURE - D:\Program Files (x86)\Square Soft\Final Fantasy VII\The_Reunion/CUSTOM/upscale/Texture/battle/
STAGE05_T02_00.png[00000827] TRACE: open_file: STAGE05.P04 (LGP:battle)
[00000827] TRACE: PERSONAL TEXTURE - D:\Program Files (x86)\Square Soft\Final Fantasy VII\The_Reunion/CUSTOM/upscale/Texture/battle/
STAGE05_T03_00.png[00000827] TRACE: open_file: STAGE05.P05 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.P06 (LGP:battle)
[00000827] TRACE: PERSONAL TEXTURE - D:\Program Files (x86)\Square Soft\Final Fantasy VII\The_Reunion/CUSTOM/upscale/Texture/battle/STAGE05_T04_00.png
[00000827] TRACE: open_file: STAGE05.P07 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.P08 (LGP:battle)
[00000827] TRACE: PERSONAL TEXTURE - D:\Program Files (x86)\Square Soft\Final Fantasy VII\The_Reunion/CUSTOM/upscale/Texture/battle/STAGE05_T05_00.png
[00000827] TRACE: open_file: STAGE05.P09 (LGP:battle)
[00000827] TRACE: open_file: STAGE05.P10 (LGP:battle)
to
[00002634] TRACE: open STAGE05.D (LGP:battle)
[00002634] TRACE: open STAGE05.B (LGP:battle)
[00002634] TRACE: open STAGE05.P00 (LGP:battle)
[00002634] TRACE: open STAGE05.T00 (LGP:battle)
[00002634] TRACE: open STAGE05.T01 (LGP:battle)
[00002634] TRACE: open STAGE05.T02 (LGP:battle)
[00002634] TRACE: open STAGE05.T03 (LGP:battle)
[00002634] TRACE: open STAGE05.T04 (LGP:battle)
[00002634] TRACE: open STAGE05.T05 (LGP:battle)
[00002634] INFO: Not present: battle/
OLAC_00.png[00002634] TRACE: open STAGE05.P01 (LGP:battle)
[00002634] INFO: Not present: battle/
OLAD_00.png[00002634] TRACE: open STAGE05.P02 (LGP:battle)
[00002634] TRACE: open STAGE05.P03 (LGP:battle)
[00002634] INFO: Not present: battle/
OLAE_00.png[00002634] TRACE: open STAGE05.P04 (LGP:battle)
[00002634] INFO: Not present: battle/
OLAF_00.png[00002634] TRACE: open STAGE05.P05 (LGP:battle)
[00002634] TRACE: open STAGE05.P06 (LGP:battle)
[00002634] INFO: Not present: battle/OLAG_00.png
[00002634] TRACE: open STAGE05.P07 (LGP:battle)
[00002634] TRACE: open STAGE05.P08 (LGP:battle)
[00002634] INFO: Not present: battle/OLAH_00.png
[00002634] TRACE: open STAGE05.P09 (LGP:battle)
[00002634] TRACE: open STAGE05.P10 (LGP:battle)
(tried highlighting the first few corresponding texture pairs for clarity, hopefully it makes sense)
I put together a zip file with screenshots, app.log, and reunion.log files from hotfixes 8, 9, and 10, hopefully it makes things a little clearer.
https://drive.google.com/open?id=1NYk5EPbnEqycUw4XIk8Zb-SCKKgzvP1q