I suspect the bug is from suffix: '.*[^!]]', where a Negative Character Group: [^] means to not match something ending with the separator that indicates a directory, according to Duplicati Syntax documentation.
Character Escapes start with backslash, so Windows directory separator backslash needs its own backslash whereas I have a suspicion that this bug would not happen on Linux. A forward slash is not special to an RE, however a backslash before a non-special character just gives the character, so “\!” in source “should” work.
Web UI workaround is to avoid “Exclude files whose name contain” in favor of something more do-it-yourself, for example “Exclude expression” or “Exclude regular expression”, then go read the documentation carefully. Checking in Commandline that the web UI did what you wanted might also be helpful until you’re used to this.
Being new at filtering, I’m both impressed and concerned by all the semantics –include and –exclude accept.
Web UI helps to cover some of that syntax complexity, however in this case it seems it didn’t get it quite right.