is NALCAP stipend income subject to US employment tax?

Hello, this is in regard to the 2023 tax year. I am a US citizen who worked part of the year (approx 6 months) in Spain as part of the Spanish government's NALCAP program. I reported the living expense stipend amount (equal to approx $3300 total over the period of time I worked there) on our Schedule 1 form as "other income."

We are married filing jointly, and my husband has a substantial income from his employment with a hospital. We've now received a letter from the IRS saying that the amount on line 8z (the $3300), appears to be subject to self-employment tax. Does this sound right? Was I "self-employed" during this period of cultural exchange/language assistance in Spain? If so, how should I calculate that? Many thanks!

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