Saving PDF to Network Share takes ages causing PDF to corrupt

We have recently seen a problem with a client saving PDF files to their network share. This has only started happening in the last couple of months.

Environment is Windows 2012 R2 Server, fully patched. Windows 10 clients, fully patched. BitDefender AV, Adobe Reader fully patched. Flat network, GB all over.

Client saves court bundles ~ 200 to 250MB in size.

Regardless of whether the PDF is opened in Adobe Reader, Chrome, Edge, the file save process either hangs or says it is complete. It then shows the file on the server share with 0bytes file size. It then increments in random file size chunk but takes between 40 and 60 seconds to save.

The app says the save is completed within 10 seconds and the user closes the app, this cancels the save which leaves a part saved and now corrupt file on the server which is not noticed until they go back to open it again.

If the file is saved locally no such issues exist and the file saves, opens and closes within seconds.

If i manually copy the file to the server it copies in under 15 seconds.

If i work with other large, non PDF files this issue does not occur.

What is Adobe doing to cause this issue?