Just did some research and it seems that the anime adapted up til end of volume 4, so maybe translator decided to adapt from what happens after the anime, so from volume 5 (chapter 25).
So Thank you translator for translating/uploading the chapters of what happened after the anime ended😁
They are a little less than three years apart, and when Eri started being Maina's oshi, she was still only a teenager (either still in or fresh out of high school) herself. So that's really not the part to focus on if you want to criticise the character for being creepy.