Plan: Cauterizer... is it even in the pool?

Dear community,

After countless of full raids completions (lost the track after finishing all the challenges 5*76/76), the last plan from Gleaming Depths that I don't know is the Plan: Cauterizer which should drop from 4th stage, with a drop rate of 5%. I've got 10+ copies of all others plans with 5% and none of this one. The statistics beat me big time on this one. Can someone please confirm if it really is the the pool or if it's somehow bugged like 3rd stage trophy, which is locked from dropping if the 4th one has been leaned before?

Thanks in advance! 🤝