Ulmus × hollandica and Ulmus × vegeta both share the same hybrid formula.

Ulmus × hollandica has been described in 1768 and is the valid name for this hybrid. POWO lists 99 different synonymes for this often cultivated hybrid between Ulmus glabra and Ulmus minor:

https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:856868-1#synonyms

Another reason for this taxon merge is that both input taxa lack a space between the "×" and the epithet. POWO should be followed consistently here, it also serves to improve readability.