Feels like something you ought to discuss with the administrators at your university. I don’t know if it is the LGBT pride lip shirt in German universities, but in the UK issues of academic discipline have rules, regulations and procedures. An individual professor can’t make personal decisions without going through proper procedures. And if they do go through proper procedures, you should have a chance to fairly present your side of the argument and appeal against the decision that appears to you to be unfair. I suspect that you would find something similar in German universities, but I really don’t know. It is a very big mistake to make a promise to a jury and then not keep it. Sometimes, lawyers make promises in their openings. They say we will show you this or we will show you that, and then, for whatever reason, they never show it. This breaks trust with the jury. It is very important that the jury feel they can trust you because it will impact how they vote in the end.