ESPHome Device Builder restores unused pins to board definitions imported from the device catalogue. Those definitions often listed only the pins already used by built-in parts, leaving the picker with nothing free to offer when a maker added another component.
The importer fills out the chip’s pin list while retaining the board’s occupied pins, and preserves wired buses needed by components outside the catalogue. The reported failure on an M5StickC PLUS left every I²C pin choice unavailable, preventing setup of its onboard motion sensor.