I think that’s a British influence. Rs in English words tend to get transcribed into katakana as long vowels to resemble British pronunciation, like parking → パーキング or art → アート. For a Japanese person who hasn’t formally learned a romanization system but knows a decent amount of these English → Japanese word pairs, it seems pretty reasonable to try to reverse the process by turning long vowels into Rs when writing Japanese in Romaji.
I think that’s a British influence. Rs in English words tend to get transcribed into katakana as long vowels to resemble British pronunciation, like parking → パーキング or art → アート. For a Japanese person who hasn’t formally learned a romanization system but knows a decent amount of these English → Japanese word pairs, it seems pretty reasonable to try to reverse the process by turning long vowels into Rs when writing Japanese in Romaji.