Just did a small test in a Leveller module and the negative effect does indeed disappear if you wait long enough.
For the second test, something odd happened when I rested before the effect disappeared on its own. A second use of Iron Body after the rest did not cause an additional loss of 6 DEX, it actually added 2 DEX to the effect.
Dex started at 13 --> 7 --> 9. And the secondary effect did not wear off.
So it looks like the Iron Body spell works as expected, if you don't rest or exit the module before the negative effect wears off on its own. Otherwise strange things happen and you risk making the effect permanent.
Thanks for your help.
Al B.