Network is affecting angular 6 application, but works with mobile data

  angular, networking, symfony

The story is simple, we have an angular application (version: 6) which display users and their events in a calendar as you can see here
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everything is working fine only for one user, when he try to use the app sometimes it works sometimes not (trust me it’s really random).
as you can see in the screenshot, events owners are listed in the left side, and the user can filter data of the right side by just selecting or deselecting the event owner, in normal case this should work, but using their network sometimes data are displayed, sometimes not.

I tried so many solutions like verifying my code, performance test etc… until we find out that if we change the network to mobile data for exemple, everything seems to work perfectly

When I verify the network section in google chrome, my backend part (Symfony 3.4) there is no problem with getting data.
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I am not that good in networking so I am really struggling, Need Help

Source: Symfony Questions