Well, personally, I don't use it for jumpers or t-shirts, but only for shirts or blouses. And in over 5 years of regular use, I've never noticed this phenomenon.

I can imagine this for T-shirts in very small sizes (XXS, XS). The underwired shape is probably too large for them (although it can be adjusted at the side with a zip).
However, I have no problems with XS men's shirts.
Tip: Only iron synthetic, woollen or viscose fabrics briefly.