In theory, shooting enough of the shorter shells will "ring" (flame cut) the chamber at the point of the shorter shell's case mouth. This has been observed in .22LR, when extensively fired with .22short ammo. And it takes multiple thousands of rounds before any problem has been seen to develope. Even then, its rather rare.
However, this is not a commonly seen situation in centerfire revolvers. In fact, the opposite seems to be true. The "crud buildup" has a similar effect to a ringed chamber (making the longer cases stick), and as noted, is common, but easily avoided by simple cleaning.
My guess is that 1)the low pressure of the typical .38SPL, combined with 2) the crud buildup, and finally 3) virtually no one shoots enough .38 ammo to matter (when it comes to flame cutting the steel) is what keeps "ringing" the chamber from being an issue in the most common short shell/long chamber situations.