We can't identify the exact cause from the symptoms pictured, but check the leaf undersides for indications of pest activity. Some of the etching-like marks on the foliage resembles that of thrips feeding damage, for example, or perhaps mites (not spider mites necessarily, but other kinds that don't make webbing). Thrips pictured in the linked page are yellow, but some species (or the adults) can be black and may look glossy (they will have the same long body shape). Management options are included on each page, which essentially involves either regularly rinsing/rubbing the pests off the foliage or treating leaves with horticultural oil or insecticidal soap. (Note: not a household soap or detergent alternative, but an actual, labeled insecticide product.)

Other factors that can potentially cause this kind of leaf damage include environmental conditions like too-low humidity and salt damage or over-fertilization. "Salts" in this sense are not necessarily sodium (though if you use softened or bottled water, it can be) but can include other minerals like calcium that can be in normal tap water and fertilizer. If not leached out of the potting mix now and then from thorough waterings (where excess water freely drains out of the pot) or periodic repotting (perhaps every year or two), the levels gradually build up in the soil to degrees that injure or kill roots. Damaged roots can't absorb enough moisture or nutrients and foliage symptoms or dieback can result. If a municipal water source also happens to have additives like fluoride, over time sensitive plants can similarly accumulate too much exposure and can suffer foliage damage. Repotting is the primary solution if that case as well, though using a purer water source might also help. (If the plant spends the summer in the shade outside, rainwater can help leach lots of residues out of the potting mix.)

Roots that stay too wet can die back and result in leaf damage, so make sure the potting mix is drying slightly between waterings and not staying damp. Despite being native to moist habitats in the wild, in cultivation, this species does best with good drainage and soil that doesn't stay too consistently wet, allowing for a bit more drying between waterings during winter.

If you see additional symptoms or think you found pests on the leaf undersides and want confirmation, feel free to send additional photos for feedback.

Miri

You're welcome.