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RWF_NOWAIT (since Linux 4.14) Do not wait for data which is not immediately available. If this flag is specified, the preadv2() system call will return instantly if it would have to read data from the backing storage or wait for a lock. If some data was successfully read, it will return the number of bytes read.
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This implies that "standard" pread/preadv/preadv2 without that flag (which is only available for preadv2) will block waiting for all bytes (or short return on EOF) and you need to set a flag to get the non-blocking behavior you're describing here. Otherwise the flag would be the inverse - RWF_WAIT, implying the standard behavior is the non-blocking one, not the blocking one.
The blocking behavior is what we were expecting (and previously got) out of io_uring, so it was an unpleasant surprise to see the behavior change visible to user-space in later kernels.