Why can a municipal bond fund go down in value?

When interest rates aren't changing, why can a municipal bond fund go down in value? The bonds owned by the fund are earning interest. There are fund expenses but they should be less than the interest earned so why is there a dip in value and how long would it take to recover assuming interest rates don't... show more When interest rates aren't changing, why can a municipal bond fund go down in value? The bonds owned by the fund are earning interest. There are fund expenses but they should be less than the interest earned so why is there a dip in value and how long would it take to recover assuming interest rates don't go up?

I watched Fidelity Massachusetts Municipal Income (FDMMX) go down in value a few percent in the last month and since interest rates are unchanged, I don't understand why.
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