PCF8575c changes address in I2C Scanner
Look at the back of the module. There are 3 groups of 3 solder pads each, marked with A0, A1 and A2. With these pads you can set the 3 least significant bits of the address.

On the images in your product link these pads are unconnected. The middle pin is the address pin. The left pad (which is connected to a trace) is Vcc, the right pad (which is connected to the big trace area) is ground. By connecting the middle pad with ground, you set the address bit to zero. By connecting it to Vcc, you set it to 1.

As the pads are unconnected, the address pins are floating. In that state, the pin is reacting to random electrical noise, thus changing state randomly. So also the address changes randomly.

You need to connect every address pin to either ground or Vcc by adding a solder bridge to the pads.