There are lots of stem plants that grow at a slight angle. They tend to grow towards where the light is most intense, I believe.

Every type of plant has its own growth rate based on the specific conditions. Some tend to not grow roots well, while some tend to send off lots of roots. It's quite variable IME.

However, it doesn't really matter, as the plants can get the nutrients from roots and from the water column. As long as they grow and are healthy, it's nothing to worry about. If the concern is keeping them in the substrate, you can use something like plant weights to help keep them down