The plugin breaks sales price

Indeed, when plugin is active,threre are no changes happened on save, it is just remain empty or shows the price, which was before.
AND no difference, is it a simple product or othe type.
BUT, you can edit the price. Choose the PRoduct pack – and there will be the field with the price, edit it and save. After that you can change the type to the other (simple) again.
In some way that field with price replaces values from others.