When four stairs are placed as in the attached image and two diagonal ones are waterlogged the water doesn't behave as expected.

Expected behavior: no matter the outside conditions the two dry stairs should get waterlogged too (water source block propagation).

Actual behavior: The other stairs stay dry if there's eg. air under the stairs.

It only works as expected when the stairs are on solid blocks.

Most likely related to MC-228434 and MC-127030
The water fluid logic with waterlogged stairs apparently does not ignore the blocked blocks (two with normal and three with corner stairs) completely.
The water can't and wont flow there but the fluid checks still seem to check if it could flow there as if it were a normal water source block.
Maybe the order of checks is wrong? As in the water source block logic first checks where it will flow as if it were normal and then where the stairs block it's flow. Reverse that order and it should work properly I assume.