Floating wood floor installations as the one described allow the lifting of planks locally. Springy floors will indicate unevenness in the subfloor. Uneven level of floorboards usually comes from deformation of joists, which normally is corrected at that level, by fitting new levelled support to existing joists and relaying the boards or having ply or chipboard installed.

Any other local corrections (below the engineered planks) may have an effect or may not.

The creaking noise is coming from looseness in boards or even more likely cracked / weak boards. Such issues, which are a problem before laying new wooden floors will often not be a problem after laying floors on top. It depends on the geometry of the issues though and therefore refitting the boards before any wood floor installation is always recommended.

There is no easy advice in such situations, especially if floors were recently laid. A possible solution (but depends on the subfloor decline) may be lifting 2 or 3 planks and fixing the surrounding planks with secret nailing close to the subfloor, so they take the level of the subfloor. Then closing back the floors. The exercise may be used as well to refit exposed floorboards as well as to reduce creaking.