@ZOS_GinaBruno I am pleading for your help! I attempted to submit a bug report to the support team regarding this issue but it seems that despite providing links to patch notes and evidence the support team assumes I am QQing about RNG of Monster Helms rather than submitting hard, cold facts about incorrect intended drop rate of Dolmen Boss drops.

As posted in the Thieves Guild Patch Notes Here: forums.elderscrollsonline.com/en/discussion/251063/pc-mac-patch-notes-v2-3-5#latest
Updated the rewards for defeating Dark Anchor bosses with unique names. Now, when you kill one and loot the chest that spawns, you will always receive one of that boss's unique items.

I have completed many dolmens since update and while I have received many drops (from my experience the drop rate for the Frost Lord, Storm Lord and Ozzozachar all drop Boss specific items in the Dolmen Chest 100% of the time) however, I have gotten the Lich of Mournhold twice since this update and I have received 0 boss drop items. Indeed, I did this boss with two other people the second time and they also confirmed they received not a single boss drop item.

Please have the drop rate of Boss Items reviewed and ensure they are set to 100% drop rate, as I do not believe every boss was updated with this RNG.