Custom entity translation issue when saving a config page

Problem/Motivation

Steps to reproduce

• Create a custom translatable entity with a translatable field
• Create an 'en' entity

*************************** 1. row ***************************
                          id: 1
                    langcode: en
                       label: Lorem ipsum
                      status: 1
                         uid: 1
                     created: 1716455825
                     changed: 1716455861
            default_langcode: 1
  content_translation_source: und
content_translation_outdated: 0
  content_translation_status: 1

• Translate that entity into another language (fr in my case)

*************************** 1. row ***************************
                          id: 1
                    langcode: en
                       label: Lorem ipsum
                      status: 1
                         uid: 1
                     created: 1716455825
                     changed: 1716455861
            default_langcode: 1
  content_translation_source: und
content_translation_outdated: 0
  content_translation_status: 1
*************************** 2. row ***************************
                          id: 1
                    langcode: fr
                       label: Lorem ipsum
                      status: 1
                         uid: 1
                     created: 1716455825
                     changed: 1716455861
            default_langcode: 1
  content_translation_source: en
content_translation_outdated: 0
  content_translation_status: 1

• Create a config page (context language active) with an entity reference field allowing to add your custom entity
• Add the entity that we created earlier
• Save the config page

At this step, the entity can't be translated anymore and the 'en' version now has an 'und' language instead in db.

*************************** 1. row ***************************
                          id: 1
                    langcode: und
                       label: Lorem ipsum
                      status: 1
                         uid: 1
                     created: 1716455825
                     changed: 1716455861
            default_langcode: 1
  content_translation_source: und
content_translation_outdated: 0
  content_translation_status: 1
*************************** 2. row ***************************
                          id: 1
                    langcode: fr
                       label: Lorem ipsum
                      status: 1
                         uid: 1
                     created: 1716455825
                     changed: 1716455861
            default_langcode: 1
  content_translation_source: en
content_translation_outdated: 0
  content_translation_status: 1