Lava can't be placed directly on steps or other places, it always floats above it

Lava Isn't meant to be able to be able to flow like water e.g over half slabs or stairs, it wasn't a feature in update aquatic like water was. I've even tried it for myself on the 1.13 versions and the exact same result appears.

If you want it to be a feature: asking on https://feedback.minecraft.net/hc/en-us would be the best place to do it.