Symex reindents expressions after each transformation (i.e., which mutates the buffer), for which purpose it uses standard Emacs utilities such as indent-region, which ultimately derive their sense of proper indentation from the major mode. If you’d like to customize indentation, you should explore the facilities the major mode offers for this purpose (as Symex does not itself define this). On the other hand, if you’d like to use your own indenting framework or another package such as Parinfer for this purpose, you could disable Symex’s automatic indentation.
(symex-tidy-after-transforming-p nil)