The problem is the host hardware Bluetooth. You can see the difference in the host macOS in

System Report -> Hardware -> Bluetooth.

On the 2012 MBP, you should see USB for "Transport" while the 2019 MBP shows UART. AFAIK, Apple has been using UART for the BT interface on the MBP line since the 2016 model.

VMware Fusion just fails silently (i.e. no error dialog but there are errors in the vmware.log) when the "Share Bluetooth devices with host" option is checked. So it is likely VMware Fusion is expecting USB and not the UART present in the host.

The workaround would be to have a separate Bluetooth USB dongle and you connect this to the VM (from the USB & Bluetooth menu); and then pair the Bluetooth keyboard (or any other BT device for that matter) with the VM.