Output extent / Same As layer not selectable in Topo to Raster Geoprocessing tool

[LaurenceConway]

Hello,

I'm having a bit of trouble getting Arc GIS Pro to select the Output extent of the Topo to Raster tool as the Same As layer option. 

The layer intended is selectable initially in the dropdown list of options, after selecting this, however the Output extent then jumps back to As Specified Bellow 

Any ideas on what may be occurring here would be greatly appreciated

Many thanks,

Laurence