It doesn't look like there's a specific question on Raspberry Pi Meta, but there's a question on Meta Stack Exchange, the network-wide meta site, which answers this. As stated in ChrisF's answer there:
You're right, putting "[SOLVED]" in the title is not the right thing to do.
If the OP made the edit then leave a comment explaining the correct ...
It could be useful to put "[solved]" into the question title if you're about to post a self-answer afterwards. That saves the effort of people trying to answer the question before the self-answer is posted.
Once the self-answer is there, one could remove the "[solved]" tag to make the question look normal.