Not saying the IC is perfect but I see some fault on your part for not assuring a stable Vin before enabling the converter.
Sorry to use such strong word, but: bullshit.
Ramping and intermittent voltages are completely normal on any system. This is the exact task for an UVLO circuit, this is why it exists, and because the need is so common, this is why it's
integrated.
If they claim they have UVLO, it must accommodate exactly the ramping and bouncing Vin case, of course within maximum ratings.
It's ridiculous to ask the user (only after sales, of course!) to provide an external UVLO on the enable pin when they claim to have one internally.
Outputting too high of an output voltage due to input brown out cannot be justified
even without an UVLO, let alone with one, unless the input bounces outside the allowed range, which probably isn't the case here.