A Microsoft file hosting and synchronization service.
After further testing, it looks like all files that have a '#' in the path name, but not the file name itself, are having this problem, and all other files are not. For example, a pdf called 'test#1' in a folder called 'test' works correctly, while a pdf called 'test' in a folder called 'test#1' does not. Given that # is not a restricted character and that the files were created and synced properly with this path, I'm not sure why that would matter. It also gives a solution, remove the # symbol from all paths, but I'd still be interested in knowing why that's causing problems.
Onedrive Version 2018(Build 18.044.0301.0006)
Microsoft Windows [Version 10.0.16299.248]
Thanks for your help.