You've not lost value though I don't think. The previous fund would have been sold at its market price and the new one bought at market price - no net difference in cash terms.
Can you just reconfigure back to your original fund choices?
Jumping in a bit late. Bonds like every other asset have a value as of now. It doesn't matter what the face value is, what you paid, or what it could be worth in the future.
A bond with a face value of £100 thats selling on the open market for £95 is worth £95. There is no but later its worth this, thats simply nonsense.
If you had any bonds in your pension then they would be valued at todays market rate, not their face value.
Shares for example always have a notional face value as well. Its the price they were sold for when listed (although thats not technically true they can be discounted or sold at a premium, but fundamentally thats the point).
You wouldn't value your shares at their notional value as opposed to the market value would you?
Pension funds (I mean DB schemes) would typically hold some bonds, they give a short term cash liquidity. The funds need some liquidity as they cannot predict every action that may be taken, the incoming and outgoings.
In personal drawdown its also worth keeping a smallish amount into bonds or cash as its your backup against a volatile market. Shares dropped 20% with COVID, you really wouldn't have been wanting to buy an annuity or sell any shares at that point.
So you hold a small balance in cash(/bonds) to see you through or at least have time to consider a large down fluctuation in share values.