MacBook USB Keyboard and Mouse no longer working via Samsung Monitor's hub

I have a USB-C MacBook (Retina, 12-inch, Early 2015). It has a single USB-C socket, so conveniently connects to my Samsung 34" Monitor which has a two port USB 2.0 hub and also powers the MacBook (Monitor Model: Samsung LC34J791WTUXEN)

This had been working fine, but I recently had a problem with my MacBook and it went away to have the logic board replaced. It also had a software update to 10.14.4 during the repair, but I don't think was previously much far behind since I generally stay up to date.

Now I find that keyboards and mice connected via the USB hub in the monitor no longer respond (I've tried all sorts of keyboards and mice from my small stock) but flash drives etc plugged in to the same hub on the monitor work OK. If I use my USB-C convertor plugged directly in to the MacBook (this has one PD connector, a USB 3.0 and a HDMI) I can plug a keyboard or mouse in to this and it works OK.

This is a strange one I can't fathom. Unless something in the monitor has broken, all I can see is some incompatibility with the new logic board in the MacBook, or maybe something has changed in Mac OS 10.14.4.

Any hints greatly appreciated. If I can borrow another laptop with USB-C I may be able to isolate the monitor.