Target Display Mode on 2012 iMac running Mojave (10.14.6)

Did 10.14.6 (or the recent security updates) remove Target Display Mode on my iMac (27-inch, Late 2012)? It was working last week with about 55% reliability (sometimes the backlight on the iMac would remain completely off, but this would resolve with many subsequent attempts), but this week after recent security updates (maybe a coincidence), nothing at all is happening when I hold the keyboard shortcut (cmnd + F2) on the iMac keyboard. (Connections are solid and the same as they were last week when this was "working.")

Thanks in advance for your help!