There are two ways that Zotero citation styles choose journal abbreviations.
Citation styles that use journal abbreviations have an option to use standard MEDLINE journal abbreviations. You can select this option when choosing a citation style for a document.
All Zotero styles have an option to enter a journal abbreviation for each citation:
If you select using MEDLINE abbreviations (option 1 above), these abbreviations will not be used. Also they will not be used if the citation style specifies that full journal names be used in citations.