@leahli
I was facing similar issue before with red/green diff appearing frequently. These are related to Genie Code Quick fix, not the normal grey autocomplete suggestions.
With Settings → Developer → Genie Code Quick Fix = On, a failed SQL statement can produce the red/green suggested correction. If the same invalid SQL is run again, the suggestion can appear again.
After turning Genie Code Quick Fix = Off, the SQL error appears normally, but the red/green diff no longer shows.
So if you want to keep normal autocomplete but stop these inline red/green fixes, disable Genie Code Quick Fix.
If the suggestions are reappearing even without re-running the failed cell, that may be a separate editor issue worth reporting.
This also mentioned in current Databricks docs: Quick Fix automatically proposes single-line fixes after code errors using a red/green diff, while normal autocomplete is a separate inline suggestion flow accepted with Tab.