Is this calcium deficiency?

Calcium is immobile, not possible to present on old growth. But magnesium is mobile and so is K, you want roughly 13:2:1 (Ca:Mg:K) if that ratio deviates too much of one can lock the others even if its there in the soil/medium. Doesn't matter if its soft, all that matters is the plant to is receiving more than its using up, potentially causing a lockout. Hard to say with little to go on. If it was lockout I'd expect symptoms all over. In a wet-dry cycle, if the substrate dries too fast (often due to high temperatures, low humidity, or small pot size), water evaporates while the nutrient salts are left behind. This causes the Electrical Conductivity (EC) to spike in the root zone, causing temporary high levels of osmotic shock due to high salinity (EC) of the medium. As soon as a plants EC skews and detects higher levels of salinity in the soil than the plant then the water thats supposed to be pulled into the plant is forced opposite direction, mass flow is crippled, mass flow is responsible for a majority of ca mg and K uptake. Plant is compartmentilizing the damage to tips of plant which tells me its semi controlled by plant, linked to mass flow problem. Gluck. Helps to get yourself a ec metre for soil (NOT a tds for water solutions or runoff)