Again I couldn't find any dupe. However, I believe that this does not have any dupe bc who thought of falling into a chain command block "treadmill" anyways.

With the use of repeat command block, it is possible to make a "treadmill" in MC (it's basically when you stand on certain block, you will move in certain direction). However, when you use chain command block as a "treadmill material", and then fall into it, you sometimes will not take any fall damage until that "treadmill" send you to stand on another block. Now in theory, this works on other blocks as well. Unfortunately, I could not reproduce this with any other blocks.

Now here's the way to reproduce this:

Step 1: Set up an "Always Active" repeat command block that said: "execute at <username> if block ~ ~-1 ~ minecraft:chain_command_block run tp <username> ~0.1 ~".
Step 2: Build a "treadmill" of chain command block that facing east, at the end of the "treadmill", place any block.
Step 3: Switch to survival, then falling into the "treadmill".

What happened: The player does not take any fall damage until standing on another block.
What should happen: The player takes fall damage right away.

Please note that this does not work even 50% of the time, if this does not work, change the "Always Active" repeat command block to "Needs Redstone" repeat command block then change back to "Always Active" repeat command block or break the repeat command block then place it again.