Impedance matching for piezo element
The transformer has the specified (rather large) bandwidth when it is loaded to some active load (resistor). This mode is assumed in its datasheet.

Your load is a capacitor. It is not an active load. So the combination of transformer and piezo element capacitance works as a resonant circuit. We do not know the transformer's inductance, I do not know piezo element capacitance. Probably it is specified on its datasheet. So it is not possible to predict the system performance. May be it would make one frequency only.

This problem has to be solved by introduction of active losses to some place. The first idea is: to add a resistor in parallel with piezo element. The starting value is about 100 kOhm. You have to check the acoustic bandwidth, if it is good, you can increase this resistor. If it is bad - you decrease it.

More specific advise requires detailed information about your piezo element.

Adding a capacitor in parallel to your primary side is a bad idea.