Since Mavericks, I can't mount ISO files on my Mac over a network
It sounds more to me as if it's a Windows issue than a macOS issue, even though I realize that it didn't start until after the Mavericks upgrade.  Have you tried any other mount tools, such as CloneDrive or PowerISO?  It may be worth trying a couple of other tools as a test.  Are you able to mount the ISO from another Mac?