mining next to cornered retaining wall unexpectedly collapses it

higher level at +5 filled to +5
retaining walls built at edges of +5
lower level next to retaining mined to +0.
next to straight wall - works like intended - can even be mined to "0 angled down to -1"
when attempted to mine next to the corner - retaining wall suddenly collapses.

expected behaviour: wall should not collapse, difference between upper and lower is exactly 5.

to reproduce load the save and wait, an excavator will come and dig thus destroing the wall section

From what I know, it's quite known that retaining walls
are highly "allergic" to ocean water.

So it's likely that they are colapsing just because of that.

have the same issue. it is more a pet peeve than anything for me as i want to make nice piers