Is hard to say but since you aren't dosing, I think you should dose with EI. Mine doesn't that. Either straight or some tends to curve towards the substrate. This is with my high tech tank with pressurized CO2 and heavy EI dosing, with dirt substrate.
I unterstand this topic is quite old but i'm having the same issue with normal hair grass(acicularis), how did you solve the issue?
Did more co2 help? I was dosing about 30-35 ppm with 0.5w x liter with T5 tubes and advanced Seachem fertilization protocol.
I also have JBL fertile soil with 4-6 cm od inhert quartz on it.