https://youtu.be/k7xnfCQi17s For video of the bug

When a minecart enters a turn at a very fast speed, it can take damage instead of breaking. (not tested with other kinds of damage) Where at a fast/slow pace, they always break immediately.

However, it seems to occur inconsistently.

To reproduce, simply create a small turning path with a cactus at a corner.
See above link for video, and the below screenshot. Despite the hitbox and model entering and passing through the cactus for multiple ticks, the minecart does not break. Yet at even just slightly slower speeds, it always breaks as the hitbox shifts inside the cactus (see end of video for stationary effect)

It could also be argued that the minecart should not interact with the corner block at all but this could cause issues for hopper minecart enjoyers.

To fix, minecarts should benefit from immunity frames and use its HP properly regardless of speed. It could again be argued that the immunity frames are working properly during the fast tests, but that would not explain the instant breaks at slightly slower speeds; in which case I will change the bug report title or its considered a duplicate.

This bug also occurs with Lava.

I highly recommend watching the above video at 0.25x speed. It helps visualize just how many ticks go by where the minecart goes through the hazard vs a slightly slower speed breaking in 2-3 ticks.