Some blocks can be placed onto farm blocks without converting them to dirt. For instance, end rods. Recently, the game made it possible to for pistons to push those types of blocks onto farm blocks without converting the farm block to dirt.
For completeness, this should also apply equally to path blocks.
Thank you for your report!
We're actually already tracking this issue in MC-161026, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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