Altmetric links up some preprints to the final published version of the research output. The important caveat is that an Altmetric record can only have one DOI. For preprints hosted in repositories where an additional DOI is assigned, we are unable to merge the preprint with the final published version due to multiple identifiers. The preprint and final published version in this case are treated as two unique outputs in Altmetric.
For example, this publication has one Altmetric Details Page for the preprint version and the final published version. The Altmetric Details Page displays the DOI of the final version, but displays arXiv as the original publisher.