Long Japanese subject fails to decode on some mailers

[riku22]

Hello.

When I send an email with a long subject such as "あいうえおかきくけこさしすせそたちつてと", some mailers fail to decode.
The Subject header is as follows.

Subject:
 =?UTF-8?B?44GC44GE44GG44GI44GK44GL44GN44GP44GR44GT44GV44GX44GZ44Gb
 44Gd44Gf44Gh44Gk44Gm44Go?=

It seems that it can be decoded normally by enclosing each line as shown below.

Subject:
 =?UTF-8?B?44GC44GE44GG44GI44GK44GL44GN44GP44GR44GT44GV44GX44GZ44Gb?=
 =?UTF-8?B?44Gd44Gf44Gh44Gk44Gm44Go?=

Is it possible to fix this?

Best regards.