In Prisma 4.6.0, changing onDelete: SetNull to Cascade results in recreating foreign key

[jameyhart]

@janpio After upgrading to 4.6.0, we started reciving the following error when migrating our database:

Error: Schema validation error - Error (query-engine-node-api library)
Error code: P1012
error: Error parsing attribute "@relation": The `onDelete` referential action of a relation must not be set to `SetNull` when a referenced field is required.
Either choose another referential action, or make the referenced fields optional.

  -->  schema.prisma:126
   |
125 |     
126 |     survey    Survey   @relation(fields: [surveyId], references: [id], onDelete: SetNull)
127 |
   |

To rectify this, we changed the onDelete from SetNull to Cascade but it then cerates a new migration that drops the foreign key and recreates it.
Is this the expected behaviour?

Our current database model:

model Survey {
    id                      String                    @id @default(cuid())
    name                    String
    survey                  Json
    reportSurvey            SurveyReport[]
    ...
}

model SurveyReport {
    id                String       @id @default(cuid())
    surveyId          String
    survey            Survey       @relation(fields: [surveyId], references: [id], onDelete: SetNull)
    reportType        SurveyReportType
    ...
}

The output from the new migration:

-- DropForeignKey
ALTER TABLE "SurveyReport" DROP CONSTRAINT "SurveyReport_surveyId_fkey";

-- AddForeignKey
ALTER TABLE "SurveyReport" ADD CONSTRAINT "SurveyReport_surveyId_fkey" FOREIGN KEY ("surveyId") REFERENCES "Survey"("id") ON DELETE CASCADE ON UPDATE CASCADE;

[janpio]

Continued here: #16228