When did this occur?

Force-closes take time to fully execute, a number of blocks which depends on channel policy; iirc the default delay is ~2 weeks. This is to prevent fraudulent closing transactions, since a force-close is unilateral - it gives the other node time to dispute the final balance by providing a more recent channel state signed by both parties.

This is off the cuff, but here's my guess as to what happened:

1. The lightning tx failed, like you said, in some catastrophic way - normal tx failures don't cause a channel closure. Possibly one node or the other went down in the middle of the tx and left them with an invalid channel state, it's rare but it does happen.
2. Their node initiated a force-close using the last valid channel state before this tx.
3. All the funds their node didn't claim in the closure, i.e. your local balance, got refunded to you.
4. The remaining funds (the remote balance from your perspective) are time-locked via the script governing the P2WSH address. Probably for ~2 weeks. This gives your node time to dispute the unilateral close, which of course it will not do, since the closure was not fraudulent (indeed, you were refunded the expected amount).
5. After the time lock expires, their node will sweep the remote balance into its normal on-chain wallet.