[Clamsoda]

10 man:

Consider the logic: If in a raid, and the amount of group members is greater than, or equal to 1, and less than, or equal to 10, show. Else, hide.

local raid = GetNumGroupMembers()
if IsInRaid() and raid >= 1 and raid <= 10 then
     self:Show()
else
     self:Hide()
end

25 man:

Consider the logic: If in a raid, and the amount of group members is greater than, or equal to 11(1 more than 10 man), and less than, or equal to 25, show. Else, hide.

local raid = GetNumGroupMembers()
if IsInRaid() and raid >= 11 and raid <= 25 then
     self:Show()
else
     self:Hide()
end

Your issue is that your IF logic isn't representing the right amount of group members. Rather than re-thinking your code logic, take what USED to work, and supplement it with the new API.