There's a possibility that your monitors are wired up where one has flipped polarity... which could reduce a mono signals' apparent response or loudness. If it was wired like that then panning would decrease the comb filtering and the missing content could appear out of no where, so to speak... seemingly as you describe. You can check your cables and you can send pulse through while watching the cones to confirm shared polarity.
A similar effect could be happening acoustically, after all your speaker electronics, because of speaker placement. Walls and shelves can make stuff disappear and then come back when you change the balance. Where are your speakers? Up against a wall, on a shelf, in a corner? It could be something like that?
It could be a lot of other things too... but those are two things to ask right off the bat.
best regards,
mike
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