SOLVED: why symfony won't save manytomany relation from form but from explicit add

Question

I've got a simple blog form with a many-to-many relation for the blogCategories. If i save it, before i call persist, the blog object has the blogCategories, but flush won't create the entries in the n:m table.

I have no error at all.

But if i call $blog->addBlogCategory($blogCategory) with a blogCategory object, it saves perfectly, which leads me to the conclusion that the relation is properly configured.

When i add a blogCategory explicitly, the object looks like this, where the first three are added by form and the last added explicitly:

 App\Entity\BlogEntry {#628 ▼
  -id: null
  -name: "65730a3121b23"
  -headline: "test"
  -content: "<p>stack overflow</p>"
  -published: true
  -preview_picture: null
  -releaseTime: DateTimeImmutable @1702038065 {#1255 ▶}
  -blogCategories: Doctrine\Common\Collections\ArrayCollection {#629 ▼
    -elements: array:4 [▼
      0 => App\Entity\BlogCategory {#1269 ▼
        -id: 2
        -name: "1869478cc6"
        -ident: "Betriebsrat"
        -blogEntry: Doctrine\ORM\PersistentCollection {#1276 ▶}
      }
      1 => App\Entity\BlogCategory {#1273 ▼
        -id: 3
        -name: "017623d368"
        -ident: "Gut drauf"
        -blogEntry: Doctrine\ORM\PersistentCollection {#1272 ▶}
      }
      2 => App\Entity\BlogCategory {#1270 ▼
        -id: 4
        -name: "13afe00644"
        -ident: "Heimbeirat"
        -blogEntry: Doctrine\ORM\PersistentCollection {#1294 ▶}
      }
      3 => App\Entity\BlogCategory {#1304 ▼
        -id: 5
        -name: "0635ff2d31"
        -ident: "IT"
        -blogEntry: Doctrine\ORM\PersistentCollection {#1371 ▶}
      }
    ]
  }
  -author: Proxies\__CG__\App\Entity\Employee {#472 ▶}
}

EDIT: SOLVED as Marleen mentioned, the field had to be by_reference=false, then it works great.