Correcting essays and the spell checking - missing colours

I am frequently giving my students assignments, where I ask them to write short essays or letters. I require them to write these online, so I can copy their works into my "feedback window", and correct them, using colours. What I have found out is that when I use spellcheck to find the most basic mistakes first and paint those words in red afterwards, the colour of _some_ of these words change back to black immediately, when I click on the "save and show the next student" button.

Any idea why is it happening? I am currently using 2.3.3+.

Thank you in advance!

Edit: also happens, when I click on the spellcheck button again, when the word is already red.