Hi Yutaka,
I do not know if the forum picks my replies as I do not see my replies.
So basically I was able to reproduce it the easy way:
1. Ensure you download and install both the MySQL and PLSQL syntaxes
2. Open or create and save one MySQL and one PLSQL file with the .sql extension (I think for one I changed what sql opens with, but unsure)
3. So you now have two saved .sql files with content under each syntax flavour open, then say you are on the MySQL syntax file, create a new file and save it, in my case the file(s) that I had set to MySQL syntax turns changed to PLSQL