Erply introduced a change in June 2024 with the release of Classic back office version 4.86, and Berlin back office version 5.78. Sales orders, prepayment invoices, quotes and inventory transfer orders now reserve inventory only for a limited time: two years by default. An unfulfilled sales order more than two years old will disappear from the list of reservations.
As a result, the reserved and available quantity of some products can change after the back office update.
This timespan can be adjusted by setting the configuration parameter inventory_reservation_max_months (sample value: 12 for one year, 60 for five years) in Settings > Configuration Admin > Parameters.
This is an optimization that makes the lookup of reserved quantities faster. A 2-year limit should be a reasonable tradeoff, considering that an unfulfilled sales order that is 2 years old is unlikely to be ever fulfilled.