Working as Designed Crimson spread Pre HM?

[MogggggBoggggg]

That is not at all what I thought you meant.
Have you used the same seed and in one placed the block and in the other not place it? Since the crimson still spreads, it might just be how it appears from your persepctive.

That's what I've tried, and that's how it is! It's only after I place the block!

 

[Lord 🦆]

It has always been like this for the Corruption & Crimson from the beginning. (Crimson since 1.2) Usually the initial world generation will be made in a way where it spreads over grass then stops spreading when it hits stone/ice/ etc. until HM. If you place dirt or dirt with regular grass next to it even in PHM, it will still spread.

The Hallow will also spread like this if you bring Hallow into a PHM world.

^

That's what I've tried, and that's how it is! It's only after I place the block!