Yes, there is a problem with files in Cyrillic.
In the joint test program, I have set the RunTimePermissions.
Added the same file in DirRootExternal with two names, "Кеннеди.jpg" and "Kennedy.jpg".
Effectively, "Kennedy.jpg" is read from DirRootExternal.
"Кеннеди.jpg" throws the error but it can be read from File.DirAssets.
I am afraid that only Erel can help here.
Anyway in a near future acces to DirRootExternal will become impossible, Google restriction.