I would avoid using IO12
"Testable: Is IO12 the culprit?
On WT32-ETH01, the only bootstrap pin of interest is IO12 (MTDI), as it can cause the voltage supplied to the internal flash to be too low if it reads high.
On the WT32-S1, the schematic shows that IO12 is left floating, with a trace bringing the pin to the exposed header, so only the internal pull-down is keeping the signal low. If you have connected IO12 to a peripheral, then perhaps the peripheral is holding the line high and/or there’s too much capacitance to drain quickly enough for boot?"