Typo3 n:m relation same table - different translation results - typo3

Question

In Typo3 9.5 I have a table for products. The products can be of normal type or accessory. Both are linked bidirectionally which works fine in default language, but if I link products to the accessory the accessory is not shown on translated product pages.
If I use the other way - linking accessory to the product - it works fine.
I have debugged the problem and found out that the resulting lines in the mm table are different for both way (see below).
What I've also seen is that in \TYPO3\CMS\Extbase\Persistence\Generic\Storage\Typo3DbBackend::overlayLanguageAndWorkspace() at the beginning of the function there is only in default language a row with the accessory.
TCA excerpt tx_products_domain_model_product.php :
'products' => [
'exclude' => 1,
'label' => 'LLL:EXT:products/Resources/Private/Language/locallang_db.xlf:tx_products_domain_model_product.products',
'config' => [
'type' => 'select',
'renderType' => 'selectMultipleSideBySide',
'enableMultiSelectFilterTextfield' => true,
'foreign_table' => 'tx_products_domain_model_product',
'foreign_table_where' => ' AND tx_products_domain_model_product.isaccessory = 0 AND tx_products_domain_model_product.pid=###CURRENT_PID### AND tx_products_domain_model_product.sys_language_uid IN (-1,0)',
'MM' => 'tx_products_product_product_mm',
'MM_opposite_field' => 'accessory',
'size' => 10,
'autoSizeMax' => 30,
'maxitems' => 9999,
'multiple' => 0,
'appearance' => [
'collapseAll' => true,
'expandSingle' => true,
'levelLinksPosition' => 'bottom',
'useSortable' => true,
'showPossibleLocalizationRecords' => true,
'showRemovedLocalizationRecords' => true,
'showAllLocalizationLink' => true,
'showSynchronizationLink' => true,
'enabledControls' => [
'info' => false,
]
],
'behaviour' => [
'allowLanguageSynchronization' => true,
],
],
'displayCond' => 'FIELD:isaccessory:=:1',
],
'accessory' => [
'exclude' => 1,
'label' => 'LLL:EXT:products/Resources/Private/Language/locallang_db.xlf:tx_products_domain_model_product.accessory',
'config' => [
'type' => 'select',
'renderType' => 'selectMultipleSideBySide',
'enableMultiSelectFilterTextfield' => true,
'foreign_table' => 'tx_products_domain_model_product',
'foreign_table_where' => ' AND tx_products_domain_model_product.isaccessory = 1 AND tx_products_domain_model_product.pid=###CURRENT_PID### AND tx_products_domain_model_product.sys_language_uid IN (-1,0)',
'MM' => 'tx_products_product_product_mm',
// 'MM_opposite_field' => 'products',
'size' => 10,
'autoSizeMax' => 30,
'maxitems' => 9999,
'multiple' => 0,
'appearance' => [
'collapseAll' => true,
'expandSingle' => true,
'levelLinksPosition' => 'bottom',
'useSortable' => true,
'showPossibleLocalizationRecords' => true,
'showRemovedLocalizationRecords' => true,
'showAllLocalizationLink' => true,
'showSynchronizationLink' => true,
'enabledControls' => [
'info' => false,
]
],
'behaviour' => [
'allowLanguageSynchronization' => true,
],
],
'displayCond' => 'FIELD:isaccessory:=:0',
],
636 = product DE
643 = product EN
644 = product FR
687 = accessory DE
688 = accessory EN
689 = accessory FR
The non-default language relation fields have the option 'Value of default language' set.
Assigning the product 636 to accessory 687 :
┌────────────┬───────────────┬───────────┬───────────────────┐
│ uid_local │ uid_foreign │ sorting │ sorting_foreign │
├────────────┼───────────────┼───────────┼───────────────────┤
│ 687 │ 636 │ 1 │ 0 │
│ 688 │ 636 │ 1 │ 0 │
│ 689 │ 636 │ 1 │ 0 │
└────────────┴───────────────┴───────────┴───────────────────┘
In this case the accessory is not shown on translated product pages.
Assigning the accessory 687 to the product 636:
┌───────────┬─────────────┬─────────┬─────────────────┐
│ uid_local │ uid_foreign │ sorting │ sorting_foreign │
├───────────┼─────────────┼─────────┼─────────────────┤
│ 687 │ 636 │ 1 │ 1 │
│ 687 │ 643 │ 0 │ 3 │
│ 687 │ 647 │ 0 │ 3 │
└───────────┴─────────────┴─────────┴─────────────────┘
In this case everything works as expected.
I think the problem of the first variant is the language overlay. For FR 636 should get overlayed with 644, but this does not happen and the product get's excluded.
But the accessory is not even present in the language overlay function.