It seems that the project dropdown/picker is limited to a hardcoded value of 250px which is too small for bigger instances where projects name prefix could be identical for more than 250px.

Same UX bug affects other similar dropdowns in the application but this one is clearly the easiest to spot.

There is an already documented workaround but that's was supposed to be only temporary until Atlassian implements a properfix.

 #issuetype-single-select, #project-single-select, form.aui .aui-ss, .jiraform .aui-ss,
    form.aui .text, form.aui .password, form.aui .textarea, form.aui .select, form.aui .multi-select, form.aui .aui-select2-container .select2-choices {
        max-width: 500px !important;
    }