Changing parent variant’s sale price incorrectly updates all variants and product template

[Jackob-K]

Module

product_variant_sale_price

Describe the bug

When editing the quantity-based sale price (pricelist rules / variant sale price) for individual product variants, the system unexpectedly overwrites the sale price of the main product template (“parent” product). Additionally, modifying the sale price of one variant causes the same price to propagate to all other variants, even though each variant should maintain its own independent sale price.
This results in incorrect pricing across variants and makes it impossible to set different prices per variant as intended.

To Reproduce

Odoo 17.0-20251121 CE:

Steps to reproduce the behavior:

1. Create a product with multiple variants.
2. Open a specific variant and change its sale price (or quantity-based price).
3. Save the change.
4. Observe that the main product’s sale price is overwritten and other variants inherit the same price unintentionally.

Expected behavior

Changing the sale price of a single product variant should only update that specific variant.
The main product template price should NOT be modified when adjusting variant prices, but it should have some symbol, so I know it won't write into invoice or somewhere.
Other variants should retain their prices unless explicitly updated.

Additional context

Behavior tested on clean database with no custom modules.