QUIC guarantees in-order delivery of streams, but not between streams. This means that each stream will send data and maintain data order, but each stream may reach the destination in a different order than the application sent it!
For example: stream A and B are transferred from a server to a client. Stream A is started first and then stream B. In QUIC, a lost packet only affects the stream to which the lost packet belongs. If stream A loses a packet but stream B does not, stream B may continue its transfers and complete while stream A's lost packet is re-transmitted. This was not possible with HTTP/2.
Illustrated here with one yellow and one blue stream sent between two QUIC end-points over a single connection. They are independent and may arrive in a different order, but each stream is reliably delivered to the application in order.
two QUIC streams between two computers