Some dupes ignore build priority

After checking, it's telling me that priority 9 tile is unreachable, but it's very reachable?

I don't think they can jump from ladder to ladder. You need a block or ladder tile between.

it more likely because of water, navigation have some issue with determining sometimes if tile is reachable. Sometimes you can workaround it by moving (using move to command on dupe) dupes to most closest tile it think it can navigate, after this (as it will be closer to destination) in some cases it will be able to do job in "unreachable" tile.

I faced this issue only for tiles with work in liquid.

4 hours ago, Vilda said:

I don't think they can jump from ladder to ladder. You need a block or ladder tile between.

they've jumped back and forth before the water started breaking the ore wall.

They currently cannot make the jump to a ladder that is two tiles away. Are you sure there wasn't anything between the two ladders before? Maybe there was a bug with an invisible block between the two ladders?