Hi Guillaume,
I'm here to help with your problem.
Can you share us the event logs on the RDS servers for any error messages or warnings related to printing or Group Policy?
And, do you use the Easy print?
Sometimes it doesn't show drivers due to network connection issues.
It is possible to install the printer driver directly on the server, so could you try that to see if that improves the issue?
Feel free to ask any questions.
Best regards,
Yu