If you manually update the Probability field (for example, to 25%) on an Opportunity while it's on one Stage (for example, Stage A, whose picklist-defined probability is 20%), then switch to a different Stage (for example, Stage B, at 50%) without saving, and finally switch back to Stage A, Salesforce Lightning keeps your manually entered Probability value (25%) instead of resetting it to Stage A's picklist value (20%). Salesforce Classic, following the same steps, automatically resets Probability back to the picklist value for the original Stage. Both behaviors are expected, and the difference comes from how Lightning Web Components (LWC) handle a "dirty" (edited) field cache compared to Classic.
This article explains why Salesforce Lightning retains a manually overridden Probability value after switching Stages without saving, and how to make Probability match the Stage's picklist-defined value.
For example, a rep sets Probability to 25% on an Opportunity currently at Stage A (20% default). They then move it to Stage B (50%) but don't save, and immediately move it back to Stage A. In Lightning, Probability still shows 25% because the unsaved edit persisted through the Stage switch. Saving the record after moving to Stage B, then switching back to Stage A, clears that cached edit and Probability correctly shows 20%.
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