I did a little bit of post mortem and found that, with the Arduino disconnected, there was about 25V on the input pin (2) of the driver.
I look at the datasheet for the TC4431 and yes it does say its a high-side driver, but clearly it is not,(likely a copy/paste error when they did the datasheets for several devices)
I noticed that too Mark. In the title it says "High Speed" mosfet driver, but then later in the text is says "High Side" mosfet driver. Perhaps they introduced a transcription error there.
I don't think so. While it is clearly not a bootstrap driver that would be used for a high-side N-channel, the datasheet only says it is "suitable" for high-side driver applications. Presumably it would be used for a P-channel FET with something like 24V.
As I pointed out before however, it's inbuilt under-voltage lockout is completely incompatible with p-channel high side switching.