Use a "run as administrator" powershell prompt and look for "stuck' instances of your program.
Get-Process -Name xwpf # display instances
If you find any, kill them.
Get-Process -Name xwpf | Stop-Process -Force # kill them
You will then need to figure out why they are hanging. What logging feature does this xwpf program have? Does your web API capture stdout and stderr from xwpf? What's in there?
I have no experience with "Dot Net Core", but from my past experience with IIS, when I would launch a sub process it could potentially run as the IUSR, the worker process identity, or the end user account (if I had impersonate=true set). Do you know what account your process is running as, and does that have an effect on it's execution?
You might want to trace the activity of that program and also the IIS worker process.
https://learn.microsoft.com/en-us/sysinternals/downloads/procmon