Look at it this way; Modbus RTU FC16 only allows you to write whole 16-bit registers. That’s the way Modbus is, the binding cannot change that.
Modbus FC22 would allow you to set individual bits in a register, but it seems like Duemmegi did not implement that command in the DFTS module, so it doesn’t help you here.
So, any kind of host system wanting to update bits in a DFTS register has to do its own workaround somehow of re-writing a whole 16-bits to set the one(s) it wants.
This workaround is outside of the Modbus protocol, not everyone would want to use the same method.
There is more than one way to do this, and Openhab does provide you with the tools to do it.
Frustrating I know, but the limitation is in the DFTS module.
Modbus does provide a simple way to read and write individual bits in “contact/coil” format but Duemmegi chose not to use it in the DFTS. (The later DFCP4 product does use simple coil and contact form.)