Full Idea
From the beginning we conceive several predicates in a single subject, and that is all there is to these metaphorical words 'support' and 'substratum'. So I do not see why it is made out to involve a problem.
Gist of Idea
A 'substratum' is just a metaphor for whatever supports several predicates
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.23)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.217
A Reaction
The question is whether the substratum is 'bare' if you remove all the predicates, and clearly Leibniz believes you are left with true essential substance (although the removal process is presumably only possible in thought, thanks to God).