I tried on my Notebook and could not reproduce this. On 1d + 2d / … the default was 10 stones. It let me set all from 1 to 50 stones. (I only tested the interface, didn’t click on create tournament to test if it got accepted thought)
On 1d + 1d /… for more than 23 stones the time setting switched to live.
I tested on my phone too. I can set 1, 5, 6, 7, 8, 10 stones for correspondence.
But if I emptied the filed (delete all numbers with backspace), the time setting changed to live. Maybe this happened in your case.
I had to replace the number by selecting and then overwriting it to get it to work as expected.
Maybe you can try this after restarting your phone? This would reduce the chance there was a temporary bug in your browser.
Can you test what happens if you use your Desktop/Notebook?
What phone do you use and which browser?
I used Firefox on Android and on my Notebook to test this.
PS: On my notebook the stones field was a number field with up down arrows allowing only numbers in the range 1…50. On Android there are no up down arrow to change the number. The input gets checked for sanity (range 1…50) too.