At one time I used to have to drive an old work car which suffered repeatedly from this. I ended up by syphoning out the screenwash tank and all the bacterial slime that was in it, then putting in dilute bleach and poking out the jets with thin wire and flushing them through.
Bought screenwash concentrate usually has a biocide in it... It prevents bugs growing on the detergent or alcohol in the diluted mix which leads to this problem.
By coincidence some time later I read a report of some research which found that a disproportionate number of commercial vehicle drivers had antibodies to Legionella bacteria, which was put down to the use of plain detergent in the screenwash bottles. Warm water containing nutrient which was stagnant for a while and then a spray generated provided a very plausible explanation for their exposure.
p.s. and it has made it to the mainstream:
https://www.cdc.gov/niosh/bulletin/2022/legionnaires-truckers.html
("Legionnaires' disease occurs at twice the rate in transportation workers as in non-transportation workers. Among the transportation workers reporting Legionnaires' disease, most (77%) were driver/sales workers or truck drivers."
and
"The bacteria that cause this illness grow better in water than in full-strength wiper fluid. When using a mix of wiper fluid and water, the bacteria can grow in the wiper fluid tank; if this happens, people may breathe in the bacteria when they mist their windshield to clean it.")
