I’ve found out that number field with Integer value can be filled with special symbols and it isn’t invalidated even with Only when ‘is valid’ option (on mobile)
For some reason it works even if I check mobile view in google chrome on desktop.
Investigated a bit more.
Symbols only - invalidated (so do empty field as well if set ‘no empty’)
Symbols with digits - making it successful again
But not when symbols are starting the input (at least in this case)
On another page of the same app - even the beginning with a symbols works with the same conditions.