Melons (and melon stems) and pumpkins (and pumpkin stems) do not grow with a block above them. 

(I know this bug has been reported before in Java, and I know that I have seen several farms built successfully with observers / pistons directly above the crop in Java post village and pillage.  However the same farm design does not seem to work on Bedrock edition.)

When bonemealed, observers do see the stem change size (from above looking down on the stem.)  However, the full stems never produce crop, and the planted stems (without bonemeal) never grow when there is a block above.  This happens both underground and above ground, so it does not appear to be a light level issue.

(I don't know if this matters, but it is a multi-player world running on Bedrock dedicated server 1.12.0.28.  However, the not-growing when covered above bug was also reproduced without the server.)

I started testing in a flat world, and it seems that observers see the bone meal update, but not the normal pumpkin / melon stem shape change update on bedrock, which I assume is a duplicate of another bug as well.