Chapter 48

(AN: Here's the explanation on how Issei lose his memory for those who's wondering:

First, he was using a forbidden technique you could say which is his new , not the old one where he's on the Scale mail of boosted gear. So obviously there will be a side effect from using that technique, and Nyarlathotep "Chaos" element which hitting him all the time also isn't helping either.

Although he started to comprehend the "Chaos" element during the fight it's still not enough so it backlashes to him by affecting his souls in the process. It's like how our body reacts to a certain poison the first time but after some time passed our body will adapt to it no?

So coupled with all that I would say losing his memories seems pretty plausible since his soul got affected in the process so his soul automatically protects itself from having the "Chaos" element corrupting it. Which in turn make him lose access to part of his memories and skills at the time.

All that happens when he fain