Weird code page behavior on old apps

Hello!

I have a weird issue that I would like some explanation, if anyone knows. At home I use MultiAVCHD to create AVCHD-DVD discs from AVI or MKV or MP4 files. These are DVD discs that contain HD video compressed with the AVC codec and can play on most standalone Blu Ray players alongside regular Blu Ray discs. When I copy paste a title with non-Latin characters (Greek) from everywhere else into MultiAVCHD it cannot display the text properly and I see question marks ???? in place of the Greek characters. But if I switch my keyboard to Greek layout and manually type the title, it appears properly. One workaround is to paste into Notepad first and then copy-paste from Notepad. Then I get the Greek characters display properly in MultiAVCHD. Also if I copy-paste inside MultiAVCHD. If I copy from everywhere else I see question marks. Why this happens? Just curious.

Also at work we use Microsoft Navision (aka Microsoft Dynamics NAV) as our ERP application running on Windows 10 64-bit. I don't know if that is related, but this also has a strange behavior. If I copy some Greek text with the keyboard layout set to US English and then paste is into some field in Navision, I also see question marks. But, if I remember to switch the keyboard layout to Greek and then copy paste into Navision, while the layout is still Greek, then I see the Greek characters properly. The same happens if I copy some text from Navision to Notepad or other application. With the layout set to Greek everything looks properly, both Latin characters and Greek characters. If the keyboard layout is set to US English I get question marks. Why this happens? Is this related to the weird behavior in MultiAVCHD? Is there any workaround? I have already set the language to Greek for non-Unicode applications.