TEXT_ICON("0") was working fine and gave this result:
Today I saw that this is broken. It maybe related to the 3rd party Service (placeholder.com) AppSheet is using. Because the result is https://via.placeholder.com/100x100?text=0 and this produces the following image:
One Workaround is to place a space in front of the number, or to use O instead of 0.
This Bug is only with 0 (zero). Any other number is working fine.