Additional product variant not visible in assigned channel when created after channel assignment #4532

Describe the bug
When a new product variant is created after a channel has already been assigned to the product, the new variant does not appear in that channel.

To Reproduce

1. Create a new channel.
2. Open a product detail page, for example “Modern Cafe Chair”.
3. Assign the newly created channel to the product.
4. Click Manage variants.
5. Add a new option, for example to the color option group.
6. Reload the page, since the new option is not immediately selectable in the Add variant popup.
7. Click Add variant and fill in the fields.
8. Return to the product detail page and confirm that the new product variant has been created.
9. Switch to the assigned channel.

Expected behavior
The newly created product variant should also be visible in the already assigned channel.

Actual behavior
The newly created product variant is not visible in the channel.

Environment

• @vendure/core version: 3.5.5

Workaround
The channel can be removed from the product and added again, but this does not seem like the intended or optimal behavior, especially because the channel-specific prices are lost when the channel is removed.