It is hard to say, it could be a bug or intended functionality. What are you wanting/needing to use the # character for? Which other characters are not working?
In GDScript the # character is used to denote a comment, which is probably why it is not working in your code. As I mentioned in this post, you can convert characters from their ASCII code if needed, so you can use characters that otherwise would be hard, if not impossible, to use in Godot.
For example, printing # would look like this:
print (PoolByteArray([35]).get_string_from_ascii());
There are probably easier ways to print the # character, but the above should work. You might just be able to use print "#", but I haven't tested so I'm not sure.
Hopefully this helps!
(Side note: Welcome to the forums!)