Bug Report: Strapi 5 - Relation Validation Locale Issue

System Information
  • Strapi Version:
  • Operating System:
  • Database:
  • Node Version:
  • NPM Version:
  • Yarn Version:

Environment
• Strapi Version: 5.8
• Database: PostgreSQL
• OS: macOS 23.4.0

Issue (only after migration to 5)

When creating a booking via the REST API with related entities (units, users), we get:

“Document with id [X], locale ‘null’ not found”

Despite:
:white_check_mark: Related entities existing in the database
:white_check_mark: Locale explicitly set (locale = ‘en’)
:white_check_mark: Locale included in the request

Repro Steps
1. Enable localization in Strapi 5
2. Ensure related entities have locale = ‘en’
3. Send a booking creation request via API

Example Request

curl -X POST ‘https://your-strapi-url/api/bookings
-H ‘Content-Type: application/json’
-H ‘Authorization: Bearer [token]’
-d ‘{
“data”: {
“locale”: “en”,
“startDate”: “2025-01-30”,
“endDate”: “2025-01-31”,
“units”: { “connect”: [“unit_id”] },
“users_permissions_user”: { “connect”: [“user_id”] }
}
}’

What We’ve Tried
• Different relation formats (IDs, Objects)
• Verified locale settings in DB
• Explicitly adding locale in request

Error Response

{
“error”: {
“status”: 400,
“message”: “Document with id "[ID]", locale "null" not found”
}
}

Questions
1. Is this a known issue with relations in Strapi 5?
2. Are there undocumented locale-handling rules?
3. Any workarounds or fixes?

Impact: API-based booking creation fails, forcing manual input via the admin panel. Any insights appreciated!