I am not quite sure if this should be a feature request or a bug report.

Describe the feature
Facades should not

Reasons why it should be considered
Stairs and slabs only let water through non-solid sides

Additional Context
Screenshots from fabric/v8.3.0-alpha.2 (latest fabric)

while the screenshots are from the fabric version, this also applies to forge.
tested in 8.4.0-alpha.1