The results you observed are actually correct and align with expected behavior.

After an audit of the calculation logic in prior versions, we concluded the way we were computing the schedule in Odoo 15.0 and prior versions was incorrect.

Odoo adheres to IFRS accounting standards, which require depreciation methods to reflect how an asset’s future economic benefits are consumed. For assets without a residual value, the declining method alone may not fully depreciate the asset to zero. To address this, Odoo compares the declining and straight-line depreciation schedules and switches to the straight-line method when it results in a higher depreciation expense. This ensures the asset is fully depreciated over its useful life.

This approach is consistent with other accounting frameworks as well. For example, the U.S. Internal Revenue Service (IRS) mandates switching to the straight-line method when it produces equal or higher depreciation compared to the declining method.

In summary, Odoo's implementation of the declining depreciation method incorporates this comparison for compliance with accounting standards. This feature was introduced in Odoo 16.0 and continues in Odoo 17.0 and 18.0.

Documentation at https://www.odoo.com/documentation/master/applications/finance/accounting/vendor_bills/assets.html