You can contact your HSA administrator to check. The most plausible explanation would be that your last contribution towards the end of the year was only received by the administrator in the first days of January 2020.
Please note that form 1099-HSA reports distributions by the HSA. Contributions to the HSA are reported on form 5498-SA.
Edited on 1/17/2020 - 12:40 PM PST
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