I think this is a completely normal effect; has occurred on every snooker table I have ever played on. If you are not actually playing a double and instead just bang an object ball cross cushions with power, on about the fourth or fifth rebound, it will effectively be parallel to top cushion just bouncing back and forth between two points henceforth. (If you were to just strike cue ball, you probably would not see the same effect because chances are, there will be spin applied to the white, intended or no, and of course, that would affect the rebounds.)
As to why this occurs, I can only speculate but I believe I have a fair idea. First off, the effect is far less pronounced on American style pool tables. The old adage about calculating a double is that the angle of incidence is allegedly equal to the angle of reflection. This is much closer to being true on an American table than an English table. I actually learned a lot about this when I originally attempted to set up my table 25 years ago. It is a Brunswick-Balke-Collender made in the USA back in the 1920's. I had cut my teeth on Rileys when I lived in England. Well, I came to find out that the rubber of the cushions actually has a different shape beneath the cloth. American design is sort of like an upside down triangle and English design is like an upside down letter "L" which basically means American cushions come to a point and English cushions have a prominent flat face. Add to that American design Snooker balls are 2-1/8" diameter, larger than English standard 2-1/16". Long story short.......I had no idea about these things when I purchased the old used table; Snooker is not exactly a thriving market over here with very little available especially when you are on a budget so you takes what you can gets. I explained to my Canadian table fitter that I was looking forward to the table playing just like what I was used to back in England. He said, "You're crazy. This is a Brunswick..." and explained all these things. So I guess a combination of me looking pathetically sad and him being a ridiculously nice guy meant that in the end, he brought my rails back home to his machine shop in Toronto and he re-worked them to accept the "proper" rubber and ball size, so in all likelihood, I am currently in possession of the only Brunswick-Balke-Collender snooker table that plays to exact English standards.
That was all an aside eventually leading to my theory of your query. In my ongoing study of the game, one of the tidbits I came across is that the contact point of the cushion to the ball is theoretically at a height of 0.62 times the diameter of the ball....think of it like the ball is a globe: the point at which the ball will contact cushion is well above the equator. Add to that the fact that on English snooker cushions, the flat face of the cushion means there is a lot more material above that contact point......as the "globe" sinks into the cushion, a lot more energy is being absorbed in the "northern hemisphere" and there is no material at all in the "southern hemisphere", just open space. You might think, "Why doesn't it rebound off the cushion at the equator?" Historically, when the rails were added to billiards table hundreds of years ago, their sole purpose was to keep the balls from falling off the edge of the table. And of course, they serve the same purpose today and more. If the rebound contact point is too near the equator, the balls would have a tendency to jump up and off the table A LOT. Remember that video with all of the spins applied and balls bouncing around? Snooker is quite a three dimensional game so the shape of the cushions are designed to retain the balls and direct the rebound back to the bed of the table if bouncing.
So the upshot is that all the extra rubber material in the "northern hemisphere" tends to absorb that energy so that the angle of incidence does NOT equal the angle of reflection. With the American pointed design, there is less material above the equator so the rebound is more accurately reflected. For exactly the same reason, it is tougher to play at the White from tight on cushion on the English table.