stronger surveillance was set on 2019-04-10. But it was not carried out, the next FI was 2019-07-22 and then 2020-04-02 (but should have been 2019-10), followed by 2020-07-15 and 2020-09-29. So the stronger interval was applied from 2020-04-02 and therefore one FI is missing to have the stronger surveillance (4 timne a year, every 3 months).