Interesting. So, out of interest, why is the same not being applied for ɢ? (When I ran it through Python's unidecode I got the roman symbol all the same).
Because 'small capital g' doesn't have a compatibility decomposition to G, but wide letter P does have a compatibility decomposition to 'normal' P. Unicode normalization kills large classes of homograph attacks but by no means all. conventions over mixing scripts from different languages stop some more, but there's no single answer.