将俄语字符转换为UTF-8时，显示某些字符的“？”

I am decoding

Subject: =?utf-8?B?W0RJXSBSVC1OTi1NVk5POiDQlNC40YDQtdC60YLQvtGA0Lg=?= =?utf-8?B?0Y8gL29wdC9lc2xvYWRlci9kYXRhL2ltL3RyYW5zbG9nINC90LUg0LjQt9A=?= =?utf-8?B?vNC10L3Rj9C10YLRgdGPINCx0L7Qu9C10LUgMiDRh9Cw0YHQvtCyIC0gT0s=?=

with mb_convert_encoding and getting

[DI] RT-NN-MVNO: Директория /opt/esloader/data/im/translog не из? ?еняется более 2 часов - OK

irrespective of

[DI] RT-NN-MVNO: Директория /opt/esloader/data/im/translog не изменяется более 2 часов - OK

Wondering what could be the cause and how to fix it ?

Thanks in advance.

I have used str_ireplace('=?= =?', '?==?', $string);

before mb_convert_encoding() it works like charm. Seems like line breaks are slicing a character in half.

Thanks anyways.