There is no scientific evidence for being able to "train your immune system". There is plenty of evidence the opposite happens. Covid especially can cause permanent immune damage.
So being exposed to certain pathogens can trigger a lasting effect, the length of which depends on the pathogen. If that effect has gone away or lessened severely over many months then it makes sense that only the innate immune response would be available against those pathogens, and the adaptive response would need to be learned again.
Of course the ability to produce the not quite right antibodies is still often valuable.