Yes, I have a same experience! Now I remember that I was taught to say 'maditta' in Korean language class when I was a middle school student, although I forgot to say so after a while.I did, just to let you know. When I was 7 years old, my school teacher told us ''mashitta' is wrong, 'maditta' is correct. ' whenever some guy said 'mashitta'.
First, I should mention that the only standard pronunciation of 것은, 옷이, 값이, 없이 is 거시, 오시, 갑시, 업시, respectively, as you seem to know already. Thus, newscasters will never pronounce that way.Rabbit, thanks for the explanation.
Now I'm interested. On these news shows where people pronounce 맛있다 like 마딨다, do they also pronounce e.g. 것은 as 거든, 옷이 as 오디, 값이 as 갑띠, 없이 as 업띠 etc.?