Does forgetting your prayers mean you don’t have to fulfill your oath to Allah?

Assalamu alaikum, so here’s a question that’s been on my mind. Let’s say a person made a promise to Allah, like “I swear I won’t smoke,” and then they fell into a really bad place and left Salah for months-like more than four months. If they weren’t really living as a practicing Muslim during that time, do they still need to offer Kaffarah for breaking that oath? I mean, they basically abandoned the prayer, so were they even considered a proper Muslim at that point? Would love to hear what you all think, jazakum Allahu khairan.