Cavity in the window lintel

Hello, last year we had a corner bungalow built with calcium silicate blocks. Apparently, KS flat lintels were used above the doors and windows (looked like a U-shaped KS shell filled with concrete). Inside, gypsum plaster was applied.
A few weeks ago, we had a pleated blind installed on a fixed glazing by the local upholsterer, screwed into the reveal from below at the window lintel. One day later, the part came off again. The anchors "slipped out." Okay, I disassembled the rest; the hole was so big that two anchors would have fit inside. I thought: you can drill the hole better than the "professional." Now here’s the thing: when drilling a new hole, the drill slipped off alignment for me as well. Unfortunately, I don’t have photos since the pleated blind is installed again for the time being.

Why the drill slipped became clear to me then: I apparently hit a reinforcement bar from below. I had the feeling that the KS block was very "soft." Nowhere else on the wall did the drill slip when drilling the hard KS blocks, but here the drill slipped half a centimeter with little force. That was already very mysterious. Then came the shock: the lintel seems to be partially hollow. I drilled a hole on the left and right outer sides of the lintel (window width 2m) and in both cases the same: a cavity. If I were to estimate: from below there were about 4cm of gypsum plaster and KS block, then about 1.5cm cavity, and then the threaded rod (whether there was concrete above it, I could no longer visually determine).

Since it’s a bungalow, not much is to be expected here, but this smells like a defect, right? I will only get anywhere with the builder if I now incur the cost of getting an expert, that much is certain. What interests me more: if the pleated blind comes off again now (instead of the cheap 6mm anchor included, I have now pressed in a Fischer FU8x40, which also had minimal play, but for now holds) – what else can I do for fastening? Yesterday I felt that every time I pulled out the test-inserted anchor, the hole got about a millimeter bigger. The cavity is too small for a cavity anchor to spread. Should I next time glue the anchor in with some kind of 2-component injection mortar (such as from Fischer) and try to inject as much as possible into the cavity?

possibly the window/door openings are simply larger and there is a cavity between the lintel and the upper edge, which would then be closed off with drywall and plastered... so not a defect per se...

With injection mortar, you can secure that well. From your description, it sounds to me as if the lintel was not properly poured with concrete. That is, of course, a defect, also because the concrete cover of the steel is missing.

: no drywall was installed in our case, I accompanied all construction phases myself, and before plastering nothing was "masked" anywhere. As I said, in the drilled hole I could quite clearly see that there was a layer about 4 cm thick made of plaster and the underside of the KS-U shell, then it turned black, about 1.5 cm of air, and then I could see the steel reinforcement. And given how easily the drill slipped, I would follow here.