I assume by “subreddit” you mean Lemmy Community?

So, you created and moderate !buyselltrade@lemmy.sdf.org , you posted something there. Someone clicked “Create Report”, and as a result you (the moderator of that community) received a report. Naturally, you didn’t ban yourself. But, somehow you were banned from discuss.tchncs.de? Do the admins of that site receive a report if one of their users reports something? Or, was there a separate report that they acted on?

In a way though, I guess this is working as intended with distributed moderation. If someone had a Hitler fan club community on their own server nobody could make them shut it down, but the existence of that community might be enough to warrant banning them and/or defederating their instance. But, I’d assume there was a separate report requesting that you be banned from their community that went to their moderators.

What’s funny about this is that the complaint was “Med spam”. Lemmy is a “pull” design, you don’t get community posts sent to you, you have to go out looking for them. So, this is a bit like choosing to go to a steakhouse and then leaving a bad review because they didn’t have good vegan options.