Probably a UTF-8 <-> Windows Mixup. Naming Items local with non-latin characters ( ö, ä ,ü e.g.) are saved normal.
When naming them in online Game, item gets Placeholder in Name.
Linked issues
Comments 2
Duplicate of MC-178.
MC-7636
Non Standard Latin Characters in item Naming
Resolved
Duplicate
1
What do you mean "in online game"? Do you mean the minecraft game playable in web browser?