On a XIAO SAMD21, USB5V is connected directly to a 5V pin, so connecting the Powerboost to a 5V pin can cause problems. The Powerboost output is set to 5.2V, generally USB5V voltage is lower than 5.2V, so current will flow from the Powerboost toward a PC (especially if a PC is turned off).
“XIAO BLE Sense” schematic may be helpful.