'Amount Base Before' value exists for the opening balance stock transaction

I've got an issue where a negative value (say $10,000) exists in 'Amount Base Before' for the opening balance stock transaction on the Average Cost Evolution report.

Eg. An opening balance loaded as a Misc Receipt worth $30,000 results in an Amount Base after of only $20,000.

How does this occur?

How do I fix it?

For time being, our stock valuations are skewed by that amount.