See here: https://i.imgur.com/TIkqEw8.png
"aberrant spectres" is all lowercase but a few lines up another monster's name is all nicely capitalized. I'm assuming this is the case for all the various Slayer monster masks.
Lower case is how it should be in this case. Where you see thing in upper case like that, it tends to be due to the sentence being constructed in script, pulling things like NPC names direct from the configs (capitalisation and all) or from a list, for the sake of efficiency and for ease of adding to larger systems in future. Not really answering you question, just explaining the discrepancy.