In the current version of the Minecart Improvements experiment, minecarts can no longer phase through semi-solid blocks like Mud and Soul Sand. This feature was preserved in the 24w33a version of Minecart Improvements, but has been broken in 24w34a.
This behaviour was not listed in the changelog, and is a heavily utilised mechanic, and therefore is assumed to be a bug
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MC-2714 is triaged and indicates that minecarts snapping through blocks is a bug, and not intentional.

While not explicitly confirmation of this being a feature vs a bug, this behaviour was featured as a part of Slicedlime's 24w33a snapshot update video's Minecart Improvements comparison segment. To a certain extent I believe this indicates that minecarts phasing through <1m blocks is not considered a bug by the development team
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Please fix, this breaks most of the cart tech

Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
Please note, that mechanics of the game may change between updates.
Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.
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this is once again an extremely important behaviour in alot of exsisting contraptions and has worked since the exsistance of rails and carts. removing this breaks a ton of contraptions and barely adds anything

This behaviour is indeed very important for tech and removing it will just detract from the gameplay as there are just no alternatives to it, please consider this a tmc sensitive issue
To be clear why this was closed as WAI. MC-2714 shows this is in fact intentional behaviour. However, design can change even if something is not considered a bug. Please keep using the feedback page, we are listening closely to feedback in regards to the experiment.

It should probably also be noted that MC-2714 is almost as old as the report about quasi-connectivity behavior for pistons, droppers, and dispensers. The confirmation of the bug status happened over 11 years ago. Consequently, players eventually expected the behavior to stay, regardless of the status of that issue report. This fix didn't seem particularly intentional, considering it was not mentioned anywhere.

Why is this even being debated? Minecarts snapping to rails has been established for over a decade now. This issue affects everyone, not just redstone players. Is changing the games mechanics really more important than giving players what they want?