Emails sent to the wrong segment in a campaign

Hi every body.
I’m quite new to Mautic but I already love it very much.
But it seems that I don’t understand totally how it works.

My Mautic version is: 2.15.3
My PHP version is: 7.2.28

Here is my problem :

I created two campaigns that I launched at the same time :

  • the first one use a segment in which I selected French speaking contacts (=whose country is set to France of Belgium - I uses filters in the segment) ;
  • the second one for contacts whose country is neither France nor Belgium (selected with filters)

Both campaigns have the same pattern :

  • I send email ONE (in French for the first segment and in English for the second one) to all the contacts of the segment associated to the campaign
  • after 5 days, if email ONE has been opened, I send email TWO (French/English), otherwise I send email THREE (French/English)

But what happened for a bunch (but not all) French speaking contacts :

  • they received email ONE in French (OK)
  • they opened email ONE in French (OK)
    - after 5 days, they received email THREE in English (KO because email THREE should be for contacts who didn’t open email ONE and KO because the French speaking contacts shouldn’t receive an email in English)
  • a couple of hours after, they receive email TWO in French (OK)

These contacts have not been added to the segment of non French speaking contacts and still they received email THREE in English. Moreover, they shouldn’t have received email THREE as they opened email ONE.
Has somebody an explanation ?
Thanks for your help.