During IBD, there is a "moving window" of 1024 blocks ahead from the current height, from which blocks can be requested from peers for parallel download. When a peer has an open slot for a block request (there is a maximum of 16 blocks assigned to a peer at a time), it is requested to provide a block from this window.
A stalling situation occurs, ...
From the Wasabi docs:
There are two ways to confirm that your Wasabi client is connected to
your own full node.
First, in the Wasabi logs. Once you receive a transaction Wasabi
downloads the block containing that transaction from your node, and in
the logs you should see: Block acquired from local P2P connection.
Second, by checking the connected peers ...
what happens to (those nodes) if they were offline or down at that moment?
When each of those nodes comes back online, it will re-connect to other nodes on the network, and request data from those nodes about the most recent blocks. Each node will then validate that data for itself, and once that is finished, the node will be back in sync with the network.