All charge pump voltage converters have a relatively high output impedance, so what you basically have is 17 V source followed an output resistor. That resistance and your load (another effective resistance or impedance) form a voltage divider. As your load draws more current, the voltage at the divider node (the output of the 7660) drops as it should.
If you want a true regulated output with varying load current, you'll probably have to shift to some other boost regulator circuit. You can search for 'boost regulator circuit' and get tons of examples. Or you can buy small modules from China for $2-$3 already assembled and tested. Or, use a 555 to drive a three-stage Cockroft-Walton generator followed by a linear regulator.
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