Credit goes to mjr4077au on ZDoom (Raze) for finding and bisecting this.
When standing in the burning E1L1 dumpster, the player takes no fire damage as of r8620. This was introduced in r8001 and the call to 'A_GetZLimits()' is causing it.
Since that commit, said call has moved to line 1523 of 'source/duke3d/src/game.cpp', but still appears to be the source of the problem.
