There exists an asymmetry in the loops that computes the intensity of H-like ions: while the loops goes over all the available levels, whether a level is resolved or collapsed is decided based on local conditions, which account for pressure lowering.
In a time-dependent simulation, this led to different lines being entered in the stack.
The bug was discovered with a sim tailored to a problem Francis Keenan brought attention to.
Reported-by: Robin Williams Fixed-by: Robin Williams