Thanks for the replies guys. No worries, I finally got a SINGLE grass block inside.
But only after painstakingly smuggling it down the stairs with pistons.
Might as well do it that way from now on. The dirt staircase method can be very time consuming depending on how it's made. The grass probably never even stopped growing, it just took a long time to spread to the next block.
For future dirt staircase use; it often works better if you make the staircase two blocks wide so the grass can spread diagonally as well.
Obviously the best way still is to use a tool enchanted with silk touch.
The grass block must have a light level of 9, and must be connected to another grass block. Make sure it's all connected properly.
If not, you'll have to stick with dirt until you get silk touch. Read more here