Next, I change the WYSIWYG editor to Basic editor. Now, my long question was excepted and there is no problem with it.
There it is. You're not doing the appropriate test. You are confusing the field length with the amount of readable characters. The basic editor takes the input as is and stores it the database field. HTML editors, as the WYSIWYG editor, add HTML around the answer.
So the word test, in order to be displayed in bold could be stored like <b>test</b> in the field. So instead of taking 4 characters it takes 11. If you have copy/pasted HTML text from the web that could even take a lot more. This is test in red: <span style="color:#FF0000;">test</span>
In conclusion, if you want to format your posts with HTML, you have to use extra space that exceeds the visible characters. Otherwise, use the basic editor.