First of all Pi-hole v3.0 FLT is very nice, the update worked very well.
The IP input fields under Settings-->Upstream DNS Servers does not work as expected.
Custom 1 + 2 shows the IP format with underscores, which works with 3-digit numbers, but not with 2- and 1-digit numbers - tested with Safari, Firefox and Chrome (macOS)
I was surprised, that custom 3 and 4 do not use this kind of "pre-formatting" mit underscores.
You need to skip the extra digits using space. I was able to enter in the address 192.168.2.1 without errors.
The 3rd and 4th custom DNS servers are for IPv6 addresses, which are more hassle to validate.
@Mcat12
Your are right with space it works, but for the last numbers block, there you can not delete the underscores, which is a little bit confusing. In my humble opinion it is uncommon to use space - normally I would expect to jump to the next input field/block with tab. I forget about the IPv6 addresses 
