i tried a clean re-installation of rimworld with medieval overhaul and his minimum mod requirement. to test the drop of human leather. and it did not drop so i took a look in some of the mod's file and found in the Add_Hide.xml :

<!-- Remove leather def from all animals -->
<li Class="PatchOperationRemove">
<xpath>Defs/ThingDef/race/leatherDef</xpath>
</li>

<!-- Add human leather def to fix human leather thoughts -->
<li Class="PatchOperationAdd">
<xpath>Defs/ThingDef[defName="Human"]/race</xpath>
<value>
<leatherDef>Leather_Human</leatherDef>
</value>
</li>

<li Class="PatchOperationReplace">
<xpath>Defs/ThingDef[defName="Human"]/statBases/LeatherAmount</xpath>
<value>
<LeatherAmount>0</LeatherAmount>
</value>
</li>

引用自 Add_Hide.xml：

<LeatherAmount>0</LeatherAmount>

i went to the vanilla file to check the original value of this line and found a :

<LeatherAmount>75</LeatherAmount>

so i change the 0amount to a 75amount and it did the job...

Sorry if i did something inconvenient here. I don't know the state of this "bug" is it recent? common? already resolve in next mod update? or not?
anyway i went through and thought sharing this might help...