The official view of the EU Commission of stamping passports of Withdrawal Agreement beneficiaries is detailed in the document “Rules for UK nationals when entering or leaving the Schengen area” on the link below. Basically provided you always carry your Article 50 card with you and display that first, it should not be an issue: https://ec.europa.eu/info/strategy/relations-non-eu-countries/relations-united-kingdom/eu-uk-withdrawal-agreement/citizens-rights_en