Heyo,
The --subs flag does not actually produce only subdomain variants when the input domain/subdomain has a trailing "." character. For example:
$ DNScewl -t fb.com. -p words.txt --subs --no-color --no-repeats | grep dev1
dev1fb.com.
dev1-fb.com.
dev1.fb.com.
I realise this might not be maintained anymore but that doesn't matter. It's a good tool that surely is still used. I've almost zero C++ knowledge and just looking at it hurts my brain so I can't point to the problematic line of code. I'm hoping this issue will help whoever might face the same problem in the future.
Easy fix is to clean your input file of any trailing "." characters like so:
$ sed -i 's/.$//' file.txt
Happy hunting!