Reference numbering wrong after adding an already used reference at the beginning of the document

I am writing an article and all references should be numbered in order of appearance (which they in general do, so that is not the question). BUT:
Lets say reference A is already cited anywhere in the document as lets say reference [12]. When I now (after this citation and after I already compiled the document before) add the same reference again at the very beginning of the document, the number of the reference does not get switched to [1] as it should be, but stays at [12]. How do I solve this problem?
I am using TexStudio, and currently write the article using the MDPI template, but the same issue occurred also when I previuosly wrote another text without any template.
What does alleviate the problem is to copy everything into a new, empty document and compile again, but there has to be a better solution.
Thank you for your help, maybe this is a very easily solveable issue but I am fairly new to LaTex and often run into issues I can’t fix on my own.