It will only show a number on larger complications were you have enough room to include a time of reading as well. Apple are obviously worried about showing old data.
It's frustrating because 3rd party apps don't get as recently updated info as they get it from healthkit. So the Apple heart rate app might have a reading from five minutes ago but the 3rd party ones complications (Like HeartWatch) don't update as reliably and will often be showing older data.