JabRef version 4.3.1 on Windows 7.

There are certain diacritics (and other symbols) that produce undesired output when generating bibtex keys. I am using [auth3][shortyear] as generation pattern. Examples with first authors' last names and year:

• Müller, 2019 -> Muel19 (4 characters instead of 3 when converting umlaut, i.e. ä, ö, ü)
• Al-Ketan, 2019 -> Al-19 (dash in bibtex key is awkward, should probably be ignored in favor of the next letter and become AlK19)
• D'Alessandro, 2019 -> DA19 (Apostrophe is correctly ignored, but the name is reduced to 2 characters when it should probably be DAl19)

Some other diacritics that I quickly tested (Ç, Â, é) are handled more gracefully, i.e. the corresponding letter without diacritic is used.

Steps to reproduce the behavior:

1. Use [auth3][shortyear] as pattern for bibtex key generation.
2. Enter one of the last names mentioned above in the last author field of a new article. Enter any year.
3. Generate bibtex key (CTRL+G)