With calcareous soils with considerable free calcium, 6,000 to 9,000 ppm Ca, & an 8 pH, it is not difficult to visualize phosphate being tied up in a calcium complex. Nothing is permanent though and this same phosphate will eventually become available to a crop.

Any ideas the time frame for the phosphate to be released?

Some authorities tell us it requires 15 to 25 lbs/A P₂O₅ to effect the soil test by 1 ppm P. For the record I plan on 71 lbs P₂O₅ to effect our soil test by 1 ppm P. Not only will the calcium chemically tie up the phosphate atoms but they can become trapped between clay platelets in our heavy clay soil. Epso Facto it is not impossible for this soil to test low in Phosphate and have an abundant supply in the plant. Turned around an acid type attractant will report a very high levels of Phosphate and the plant have a positive response to phosphate fertilizer.

Here at least soil testing for phosphate is a WAG at best.