“What does leaven symbolize in the Bible?”
Lots of things, none of them good. Let me try to make that case.
Symbols can be tricky things. Where our Bible interprets a symbol, we generally have no difficulty at all, but there are also times when the meaning of a symbol is open to question because the Holy Spirit has not told us plainly, “This signifies that.” Leaven was unacceptable to God in some contexts and acceptable in others. We will need to find an explanation for that.
The Old Testament uses leaven as an illustration. The New Testament unpacks its meaning.