Screen Flow Lookup Field Value Is Cleared When a Validation Error Fires on Form Submission

o preserve the Lookup field value across validation events, set the Record ID input of the Lookup component to store its own output value — specifically, use the component's auto-output variable such as {!myLookup.recordId} (using the format ApiName.recordId).
This configuration tells the flow to store the selected record ID back into the component's input on each render, so that when a validation fires and the screen re-renders, the previously selected value is restored.
Steps to configure:

1. Open the Screen Flow in Flow Builder.
2. Click on the Lookup component on the screen element.
3. In the component's input properties, find the Record ID input field.
4. Set the value to the component's own auto-output record ID variable (for example, {!LookupComponentApiName.recordId}).
5. Save and activate the flow.

Note: By default, Lightning runtime screen components have no memory between screen navigations. If a user enters a value, navigates to another screen, and returns to the component's screen, the user-entered value is lost. To enable a flow to retain an attribute value, explicitly set the attribute — the flow then stores the value automatically. If you store values manually, use the attribute's output value and store it in a variable.