Thank you for the recommendations. I have increased the simulation time from 5*10^5fs to 10^6fs and changed the PML from the "standard" profile with 64 layers to the "stabilized" profile with 256 layers, which has removed the late time divergence. However, this has introduced new absorption lines in the T spectrum which were not present before, such as these:

I assume this is due to the PML change. Can you recommend other PML settings?