This has happened to a few of my colleagues who pay into the additional voluntary contributions section of our pension.
The "Lifestyle" option used to progressively move the pot from a growth fund to a deposit fund (ie cash) over your last 5 years of work, which left you with 10% growth/90% cash on retirement. However, the funds were re-organised last April, and one little detail was that funds were put into bond funds rather than cash. This little change, by my estimate, has cost some colleagues almost 30% of their AVC pot. As they are planning to retire this year, there's no way they can get that back.