Doesn't work with MF0UL21 formatted as legacy MF0ICU1
The app says it works with MF0UL21, among others. I was able to successfully write a 16-page first-gen payload to a MF0UL21 (which is backwards compatible), however upon attempting to read it back, the app now says it is not a valid ultralight tag. But the MTools BLE app happily reads it as one.