I think it may be due to this https://nitter.kavin.rocks/RajeevJayadevan/status/1651796168777830400#m

People with Long COVID made more antibodies after subsequent vaccination, compared to people who had fully recovered from COVID. (More isn’t necessarily better)

This difference was sustained over time.

This indicates persistent immune activation.

So it points to covid causing autoimmune disease, which there is plenty of evidence for. In cases like this the vaccine may aggrivate it.