Is the USB-C 20W Power Adapter Dual Voltage?

My iPhone SE2020 came with the USB-C cord that required an Apple USB-C 20W Power Adapter, which I purchased when I got the new phone. I recently traveled to Ireland, where voltage ranges from 220V to 240V. I brought along both an adapter and converter, to use with our iPhones, Apple watches, travel flat iron and my husband's CPAP machine.

While the Apple watches charged just fine, and the CPAP worked well, the travel flat iron would not stay on for more than 2 or 3 minutes, then shut off and would not heat up. The worst part, is that it fried my USB-C power adapters. Why is this? Trying to read up on it, and it's not very straightforward, but I am guessing that the USB-C is dual voltage, therefore did not need the converter and only needed the adapter?