If post contains about or more 55000 symbols, then BBcodes (b, i or quote for example) cease to work

I posted details on russian support forum:
http://www.phpbbguru.net/community/topic16064-375.html#p293331
But i don't know, whether they have informed you or have simply ignored my message as there.

IMPORTANT NOTES:
1) Bug exists only if the symbols are russians (maybe for other languages except for english also, i don't know, because didn't check). For english all works normally even if there are 100000 symbols (didn't check for more)
2) I haven't revealed exact regularity of bug appearance. In one cases it works, in others - not, as test file that i attached below or quote in the beginning of this message, for example:
http://www.ninjaturtles.ru/forum/viewtopic.php? p=231068#p231068

Also, at attempt to edit this message, it becomes empty