it appears to combine the traits of both (c) and (d) types from the list Passive Participles | Russian language grammar on RussianLearn.com

since there're tons of exceptions from the rule in Russian this could be one of them

then i believe the rules were defined in hindsight to put the language in a kind of logical framework, but since the language itself didn't develop according to any set of strict rules or strict logic, exceptions occur and more often in a such complex language as Russian

also the -ут of the infinitive isn't dropped in the participle

another one is подчеркнуть - подчеркнул - подчёркнутый

it also may be related to consonants junction -веРН- and -чеРКН-


another idea is to check whether imperfect infinitive of this verb can receive ё

подчеркнуть - подчёркивать - fits

повернуть - поворачивать - seemingly doesn't fit, but there's a folk form повёртывать, which is admittedly rare and doesn't go with every prefix