We have had this issue on and off since 1.16. After navigating to the Discovered hosts page, then clicking on provision next to a host, you get the Select initial host properties screen:

We have had multiple users say that clicking on the Customize Host button does nothing. It seems to be hit or miss. Sometimes you can refresh or reload the page and then it works. It seems to only happen to brand new discovered hosts.
So far I believe most users have been using Chrome at the time.

I've delayed writing this issue for the last 6-8 months because I can't always reproduce it and the web console shows no errors. However, it does happen about 20% of the time and with 400+ users that ends up being a lot of time.

The current work around that works every time is to click on the host name itself, then on the top right under "Select Action" choose Provision. This works 100% of the time.

Not sure what I can do about this. Im trying to find some sort of pattern or error. I'll add to this ticket when it happens again which will likely be soon.