Tenant migration’s tenant oplog applier doesn’t handle retryable internal transaction writes properly.  And, queryable encryption (QE) uses retryable internal transaction machinery. This means, retrying the QE write ops on recipient after migration commit, can cause duplicate commits. So, as short-term fix, server should block QE when run in Serverless mode in 7.0. ( see SERVER-77236 for long-term fix).

EDIT: We have decided to go with the short-term fix as “fail the tenant migration if the recipient sees any donor QE oplog entries <= 7.0”.