Frostwalker enchanted boots on an Armor Stand only affect water if pushed by a slime block; previously it was possible for Frostwalker Armor Stands to affect water while in a minecart.
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Adding an image for clarity. This setup would've resulted in the water freezing when the minecart moved on 1.13, but no longer works on the current beta.
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Tested and confirmed in 1.14.30 using a setup similar to the one shown in OP's picture:
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Duplicate of MCPE-41398

@Bernardo Gomes Negri this is related but not a duplicate of MCPE-41398 because MCPE-41398 says nothing about minecarts, and the OP here says that frost walker in a minecart was working prior to 1.14.
Thank you for your report!
However, this issue has been temporarily closed as Awaiting Response
Is this still an issue in the latest version? If so, can you please add it to the affected versions (or mention it if you are not the reporter).
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Issue still occurs in 1.16.20, even adjusting test based on fix to MCPE-41398.

Frost walker armor stands also do not work when pushed by a piston without a slime block.