Let me answer your questions in order.
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The IPUMS CPS team is reasonably confident that the cause of this discrepancy is the 2018 split panel survey. In 2018, about 5,000 households in the ASEC sample were randomly assigned to receive an alternative questionnaire which replaced the health insurance coverage questions with the ones that were used prior to the 2014 CPS redesign. According to the accuracy statement, about 4,200 housing units were determined to be eligible for interview and about 3,500 interviews were obtained. This lines up with the 8,665 observations from the 3,494 households that constitute the gap in sample sizes. Since these households received the same March Basic questionnaire, but a different ASEC questionnaire, they are not included in the ASEC files harmonized by IPUMS. The team is considering adding these files at some point in the future.
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MARBASICIDP is currently not available for March 2021. The CPS team is currently working on making it available to users. As an alternative, you can use CPSIDP to link individuals between the March Basic and ASEC surveys.