As u may know, there are two different enzymes that break starch down and eventually produce fermentable sugars. Alpha amylase works and survives at higher temperatures and breaks sugar chains (almost) wherever it happens to sit along the chain. This will efectively form shorter and shorter dextrins (glucose chains) and to a small degree also fermentable sugars (when it happens to sit at the end of the chain that is rare because the chains are i initially very long).
The second enzyme, beta amylase works at lower temps and also denatures lower. It only sits at the very end of the chain and thus always produces a fermentable sugar molecule when it catalyzes a reaction. Thus, beta is more effective at producing fermentables, but only when there are enough free chain ends (produced by the alpha) that it can use.
So to get an effective conversion all the way to sugars we want to have both enzymes active for the full length of the mash. Alpha will rapidly denature at around 170F and beta already at about 160F. These are not very exact values as full denaturation always takes some time and enzyme stability depends on physical conditions such as ion concentrations, pH etc. Denaturation will also occur at any lower temperature with enough time.
If you start mash at the 156F and continue for half an hour, especially alpha will be active and beta will struggle. In fact a good bunch of beta is probably denatured after some activity at 30 mins. Then you decrease temp and most of the alpha will continue, but it is not so active at lower temp and would take a long time to produce fermentables by alpha. Some of the beta has probably survived and yes, it will chew some dextrins and produce fermentables before it denatures completely. But this system is not the most effective way to produce a highly fermentable wort as you would kill so much of the beta too soon. In fact single step mash at 150-151F for a full hour is quite efficient. A little bit lower than that (148) can make it a bit more fermentable, especially if you are willing to give it a little bit extra time (90-120min or so). More fermentability means higher abv but thinner body and mouthfeel. If you want more body, you can mash a bit higher in a single step for about 60min. Most of the time single step at a constant temp will be enough but there are beers that may benefit from protease rest or decoction mashing. Increasing mash time will generally give you a little bit more fermentables given that temperature is not so high that enzymes get killed too early.
So to your question, sugar does not get COMPLETETELY locked in 30mins at 156F because there are some active enzymes left and these enzymes can produce some more fermentables but yes, the maximum fermentability that you can reach is already limited when you start much higher than 151F cause you have permanently lost some valuable beta activity and as you drop temperature, denaturation still proceeds and the enzyme activity will stall sooner because of the initial heat that has unnecessarily diminished the amount of active enzymes.
