Hi @John Minihan ,
You may collect diagnostics to gather logs for more details to troubleshooting the error.
resolutions-with-cdc-for-oracle-attunity
Here is a same issue kb4500595-fix-cdc-source-preview-fails-with-the-given-key-was-not-present-in-the-dictionary-error which has been fixed in SQL Server 2017 CU15, so it is really strange about your issue.
I'd like suggest that you submit the issue at https://feedback.azure.com/forums/908035-sql-server.
If the issue mentioned by customers for many times, the product team may help deal with it. Your feedback is valuable for us to improve our products and increase the level of service provided.
Regards,
Zoe
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