Let's isolate why your employee can't enter their PO Box address.
I've tried setting up an employee on my sample file, and I was able to enter their mailing address. That said, let your employee run some troubleshooting steps to fix this weird behavior.
To start with, let your employee enter their PO Box address in an incognito window. It is the best place to identify what causes the email subject to change. You can use these keyboard keys to open one:
For Google Chrome: Ctrl + Shift + N
For Mozilla Firefox: Ctrl + Shift + P
For Microsoft Edge: Ctrl + Shift + P
For Safari: ⌘+ Shift + N
If the details show up, they may continue to complete the task on the regular browser and clear the cache.
They can also switch to a different browser like Google Chrome, Mozilla Firefox, Safari, or Microsoft Edge to isolate the issue further.