Lid popping when I checked for seal the next day

As long as the lid doesn't pop up again, you're OK, I'd say.

IMO the reason that this one didn't pop on it's own could well be because that jar was more full. More full means less air. It's the contraction of the air as it cools which creates the vacuum that makes the lid pop. If there's not enough air, its likely that insufficient vacuum will be generated to cause the pop.

The fact that it did pop when you pressed it re-enforces this. If the seal was bad, it would have let enough air in that it wouldn't stay down when pressed.

Note that I've never made jam/chutney, but am fairly well versed in Physics!

You should either store that jar in the refrigerator or reprocess it with a new lid.

The vacuum formed is not going to be as strong as your other jars and may be temperature sensitive. It may stay sealed on the shelf, but it may not. I wouldn’t risk having to discard the whole jar.