"Invalid custom field record type"

I'm trying to do a CSV update for a new custom field I've created for the Customer record. This is setup as an Integer Number type of field, and my csv is simply the Internal ID and a number field. What is strange is that of the 365 Customer accounts that I am trying to update, 46 of them DID update while the rest did not. Does anyone have any idea on why my csv update would fail when I can simply go into the UI and update the field without any issue?

I previously had the same thing happen when I added new custom fields and tried to update them. I tried several truly random things to fix it and it seems to have fixed itself in the end. It doesn't seem possible that the error is actually related to the specific field I'm trying to update, and I've started to wonder if this isn't actually an issue with other fields on the record. We do currently have a large number of fields so perhaps the record is simply too cluttered? I would appreciate any assistance as I don't relish the prospect