Flow Designer promotes Date type inputs to Date/Time type inputs in some scenarios. This is observed by creating a trivial Action with one input, a Date, and then using this action and its input inside a Flow. When the Date is used within a Log Activity within the Action, it is promoted to a Date/Time input. This has been observed only occurring when using the Action from a Flow, but it could present itself in further similar scenarios.
The Date/Time type change can lead boolean comparisons to unexpected results when mixing Dates and Date/Times, and can result in unexpected String conversions, as Date/Time inputs are converted to Strings such as "2020-12-29 16:00:00", while Date input is converted to a String such as "2020-12-20".
The Log Action produces a String like "2020-12-29 16:00:00" instead of "2020-12-29".
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As a workaround, it may be possible to use an inline script to handle input explicitly as a Date.