Delirium Atlas Node preventing Unique Bosses spawn

I've been running T15 maps and have encountered a unique boss on my Delirium maps every now and then. But since taking the Delirium Node 'They're Coming to Get You' - which should increase their spawn chance by 25%" - I haven't encountered a single Unique Boss in over 20 T15 maps.

Either I'm extremely unlucky or this Atlas node is broken.