Here is a quote from just one of the studies that have been performed:

"A recent study suggested that virus-mimicking anti-idiotype antibodies that are present after infection or after vaccination may potentially explain some of the long-COVD symptoms [21]. Therefore, it is plausible to hypothesize that vaccines may prolong the existing symptoms of long COVID by stimulating the immune response. A recent report even suggested that vaccines may rarely cause long-COVID-like symptoms (without a history of infection)"
https://pmc.ncbi.nlm.nih.gov/articles/PMC10849259/#:~:text=Conclusion-,In%20people%20who%20have%20already%20contracted%20COVID%2D19%20and%20now,year%20after%20the%20initial%20infection.