Is there a discrepancy about Tomura's Quirk?
As seen with different quirks as well, the user does not always have full control over it. Something which is also seen very well with Eri's quirk Rewind.

So much rather than assuming it's a discrepancy in the plot, the assumption that Shigaraki has no/clumsy control of his quirk is more likely.

Another factor at play might also be the speed at which the decay works. Through out the manga we have seen that the decay can set in almost instantly, but at occasion also act's slowly. Both of which might also be the case due to him not having proper control of his quirk.