For context, it's important to keep in mind the population density in former DDR regions is very low. Young people often flee towards bigger liberal cities, as soon as possible. Pronounced in eastern Germany, the AfD is a national problem.

In short: Propaganda. Germany is a major target of Russian influence. The AfD itself is heavily funded by Russia. There is no "old love" situation, Russia isn't representing socialist ideals or anything. (Russia wasn't loved back then either, btw.) Quite frankly, AfD followers often are just misled and detached from reason (e.g. objectively voting against their own interests). If you talk to them they often entertain some really fucking wild ideas and conspiratorial thinking. Of all parties in Germany, AfD has probably the most successful social media campaign, especially on TikTok. Mind you, the AfD's "not our problem"/"do nothing" position is aiding Russia in Ukraine. It's easy to put a nationalist/antisemitic spin on it.

Now, the leftist party is another story. There you find a completely misguided "old love" base, which is dogmatically "pacifist" and anti-NATO. They really should check the values Russia represents these days...