I've now seen that it isn't just the first letter in the file name, but all letters that change from an uppercase letter to a lowercase letter. The first file had only one letter in the name A-40-01, and this changed to a-40.0-01. I have since noticed that files such as A-40-F-01, change to a-40-f-01.

 

I've also noticed that this case change only happens to the model space drawings when they are edited in place from inside the plot file drawings (layout space drawings). The names of these model space drawings do not change however, when they are edited in place from other locations.