Thanks for the reply.
This morning (before reading your reply) I thought I better test my setup and try and see why my previous version of DOpus (v8 32bit) did display the .htaccess files.
It now seems the problem was due to how things were configured with the new install of DOpus (v10.5 64bit). I had imported my previous settings into DOpus, expecting there to be no problems.
I have now uninstalled and reinstalled DOpus 10.5 and, this time, NOT imported my previous settings. This default install DOES work correctly, i.e. ".htaccess" and any other nameless files (or files named with a dot and no extension, if you prefer), e.g. a file named ".txt" do now display when "Hide file extension in Filename column" is enabled.
I believe that the original problem is related to how I had DOpus configured to display files in Details view but with extensions hidden in the Filename column. I had a button that toggled the extensions visibility in this column which I think must be based on this method: Show / Hide File Extensions
I will try incrementally reconfiguring my setup and see if the problem arrises again. If so, I'll report back in this thread in case it is helpful to understand the underlying issue.
Thanks again.