One-to-first joins are case-insensitive. That's inconsistent with one-to-many joins which are case-sensitive.
ArcGIS Pro 3.3 — Add Join (Data Management)
Optionally, set the Join Operation parameter to Join one to first to prevent duplicate object IDs.
The Join Operation parameter has three states to adjust the cardinality. The default is blank and will allow the data source to attempt a one-to-many join. The Join one to many option will work only on specific data sources that have an Object ID field. The Join one to first option will use the first match in the table, which may result in different outputs if the Object ID field is changed or the workspace the table is copied to changes. One-to-first joins are not case sensitive; one-to-many joins are case sensitive.
To make joins more intuitive, make them all case-sensitive.
Why aren't one-to-first joins case-sensitive?
