Missing Diacritics in Relationship Fields

Hello,

I have trouble displaying special characters inside the relationship fields of my CPTs.

My CPTs names are:

• Přihláška
• Místo
• Termín kurzu
• Kurz

All of them, except the last one have troubles when displayed in the relationship fields. For example instead of "Select a přihláška" I see "Select a p?ihl??ka".

Strangely enough, in some cases the diacritic is correct and sometimes not, e.g. "termín kurzu" is sometimes displayed correctly and sometimes like this "term?n kurzu".

It seems that this problem is related solely to the Relationship fields, everywhere else the names are ok.

Could you please look into it?

Thanks
Petr