Steps to reproduce:

1. Set the input of your OS to Katakana
2. Create a user with a name consisting of Japanese characters. For example, Lachlan can be made by typing: 'ro ku ra nn', spaces included.
3. Use the Insert menu dropdown to create a mention
4. Type in the name. Note that no suggested name is given in Firefox. Chrome does provide one.

The first mention was generated in Chrome, and the second is my attempt to generate one in Firefox

Other

I've tested this with other multi-byte languages such as Hebrew and Arabic and was unable to replicate the problem.

Workaround

1. Write out the name
2. Copy it
3. Swap your input source from Katakana to English
4. Make a mention
5. Paste the name into the mention