Summary:
Bone mealing an azalea/ flowering near 2 tall blocks (solid or transparent) prevents it from growing into a tree (whether the block is right beside it or diagonal)

Blocks that prevent azalea/flowering from growing:

• Sugarcane (2 tall)
• Bamboo (2 tall)
• Extended Piston Head
• Double Tall Grass
• Large Fern
• Twisting Vines (2 tall)
• Small Dripleaf
• Big Dripleaf (2 tall)
• Lower Half Slab (on the second block)
• Trapdoor (on the second block)
• Stacked Signs On Top
• Glass Blocks
• Any Soild Blocks (2 tall)
• Pointed Dripstone (2 tall, bottom)
• Stacked Carpets

How To Reproduce:

1. Grab a azalea/flowering azalea.
2. Choose any listed for testing.
3. Place it right beside the azalea/flowering azalea or diagonal.
4. Now bonemeal the azalea/flowering azalea.

Observed Results:
The azalea/flowering consumes the bonemeal but doesn't grow into a tree.

Expected Results:
Azalea/flowering should grow into a tree.

Note:
I don't know if this is intentional or not.