Onewire sensors like the DS18b20 don’t like to have their bus power cycled on and off. Back when I last experimented with them (with a different sketch not related to ModularSensors), if you’ve got the sensor connected to the switched power supply, you had to do “DSsensors.begin();” each time the logger woke and powered up the switched power bus. Only then will the DS18b20 be seen by the Mayfly. So I just put that command in the loop right after the Mayfly wakes up from sleep.
The easy solution to this is to just connect the DS18b20 to the Mayfly’s D5-6 Grove jack and put the power-select jumper to “3.3v const”(it’s default position) that way the Onewire sensor is constantly powered and won’t “disappear” after the pin 22 switched power is cycled. Having a constantly powered Grove jack for I2C and Onewire peripherals is exactly why I designed the Mayfly’s D5-6 jack power selection options.