There’s a long list of threads regarding this problem, example: Unit costs (problem) - #142 by CEOHaaks
On older versions of Manager, before unit cost calculation was changed for the nth time, you could go into obsolete feature and re-enable automatic cost calculations. After FIFO inventory was introduced, you had to manually recalculate each time.
The main concern from the developer was that automatic calculation would be too slow, but many users said it shouldn’t be a problem since every other accounting program can do it. The calculation workflow has been improved several times since, but it’s still a manual task.
Basically, you’d lock your books to prevent prior COGS from changing, but the inventory cost remains a manual process until the developer takes a new approach.