The Bug
Many plants in game are able to be set on fire or burn though some plants aren't able to be set on fire or burn.
Affected Plants
Cactus
Sugar Cane
Sea Pickle
Pumpkin
Carved Pumpkin
Melon
All Crops/Stems
Reproduce
Get a Flint and Steel
Get one of the plants listed above
Light a fire near or on them
Observed Result
They don't burn
Expected Result
The would burn
I can confirm this behavior.