Interesting find!
I tested an import and using Encounter.measurement0 placed the data under the correct Length field. When I skipped 0 and entered Encounter.measurement1 instead, it was saved under the Water Temperature field.
Our Bulk Import help docs do mention that numbering must start with 0. I’m sorry if the cheat sheet I referenced in your previous post didn’t make it clear that numbering should start with 0.
You’ll want to update your spreadsheet, delete the import, and re-import the updated spreadsheet.