there is a bug in np++ if you safe a textfile
Normal you give a document title and np++ adds the .txt but if the document title has something included like title v3.3.0 he safe the file without the ending .txt if you don’t add it by hand
I think the rule is that “.txt” is added if the file name does not have a suffix already. In your case, the file has the suffix “.0”.
Personally, I would rather NP++ didn’t add the .txt suffix at all (I don’t know if this can be prevented) because I sometimes want to create a file with no suffix at all.
Have you tried the option:
Settings > Preferences > Default Directory > Use new style save dialog (without file extension feature)