Well if it shouldn't fill what's wrong with it, as it was obviously full.
A properly functioning drain, of any sort, will empty immediately. The fact that it did not means it is blocked. However the fact that the water eventually went away means that it is not totally blocked. As you say you can see soil in it, it is most likely that the water has gradually percolated through the soil.
As was said earlier a soakaway (traditionally) was a large hole filled with large bits of rubble with the drain discharging in to this. The rubble takes up c. 2/3 of the volume and stops the hole from collapsing. The water goes into the other 1/3 and gradually drains into the ground.
There would have been little if anything to stop rubbish (soil, leaves, basically anything in the garden) from getting carried in to this and eventually filling up the 1/3, leaving no room for the water. As more rubbish gets washed down, the drain leading to the soakaway also starts to fill up. This takes decades, from 1974 to now is plausible.
If you have a soakaway then it has reached the end of its life and needs to be replaced.
Ordinary surface water drains also get rubbish washed into them and can be blocked. Once a partial blockage happens, for whatever reason, then other rubbish will catch on that and gradually obstruct the whole drain.
If it only serves your house then it is your responsibility, so waiting for maps will not help to resolve the problem. In post 10 you do not show the direction the drain runs. If it runs toward the road then it
may be a surface water drain, it it goes in any other direction that is less likely.
In your position, my next step would be to take the advice of
@freddiemercurystwin and use some drain rods to clear more of it. A set of them will have a sort of corkscrew head and you can use that dig into whatever is in there and pull it out. This should give you more information about where it goes and what it is.
The other option is to get a drain cleaning company to investigate. They will have high-pressure jetting equipment to force rubbish out of the drain and an inspection camera to go down there and see what is going on.