Visual assessment is often easy but the only way to be 100% sure it to have a tissue test performed by a reputable laboratory. There is also a correct procedure for collecting and submitting the sample along with its interpretation.
Deficiency is best corrected by using one of the foliar applied manganese liquids which are available at various concentrations.
So is it worth treating this “transient deficiency?” I would have to say yes. With no corrective measures this deficiency can become serious, reducing plant growth, stopping development of the harvested crop or just killing plants completely.
Why does this deficiency occur? Lots of reasons mainly associated with the soil texture and its properties although soil and crop management influence severity. One thing for sure, don’t bother putting it on the soil as a fertiliser your soil is unlikely to be short; it’s just unavailable to the plant.
The images below are showing a deficiency in a Spring Barely Crop