The source says the normal route wants three opened water taps or mechanisms. A shortcut can get you to the top with only one mechanism active, but that state can leave the battle untriggered.
The useful player action is to treat this as a stuck-state reset: find the side gap, dive in, open the missing mechanisms, teleport out, return, climb again, and check whether the second cutscene starts the battle.
Normal route
Open three water taps or mechanisms.
Bad shortcut
One mechanism can reach the top but no battle.
Recovery cue
Side gap plus dive route back inside.
Reset claim
Teleport out, return, second top, battle.