The bug
When a piston pushes a stair into water, the block remains dry. The expected behavior is the stair becomes waterlogged. Pushing a waterlogged stair into another water source block also makes the stair dry. See the attached images.
Also waterlogged block pulled doesn't leave water behind. Water vaporizes instead of staying unaffected.
Linked issues
is duplicated by 6
MC-272474
Piston pull or push a waterlogged block, water disappear.
Resolved
MC-266868
Pulling a waterlogged grate deletes the water
Resolved
MC-177504
Water logged blocks do not remain water logged when moved by piston.
Resolved
MC-197130
Waterlogged stairs lose their water when pushed by pistons
Resolved
MC-185281
Waterlogged Block When Moved with a Piston
Resolved
1 more links
I can confirm. (18w21a)
See added images of the experiment.
(top to bottom) Result
Not waterlogged into not waterlogged (dry)
Not waterlogged into waterlogged (dry)
Waterlogged into not waterlogged (dry)
Waterlogged into waterlogged (dry)