Many of them are seeking full-time work but cannot find it. This is tracked via what is called the involuntary part-time rate, which has been rising:
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Signs of Cyclical Weakness in Part-Time Employment? - San Francisco Fed
Involuntary part-time employment—the share of people who work less than 35 hours per week but want and are available to work full-time—has increased since 2023. While this contrasts with steady declin...