Thomas, who represents the Llantarnam ward in Torfaen, hopes to stand as a candidate at the Senedd election in 2026.
Reform believes it will win several seats in Cardiff Bay.
On the social media platform X on 6 February, Thomas said he was “absolutely thrilled to have the honor of officially opening the stunning, futuristic new [Lidl] store in Cwmbran today”.
He posted a picture of him holding a large pair of red scissors, alongside Torfaen’s other two Reform councillors, Jason O’Connell and Alan Slade. Other pictures showed him looking around the store.
A similar message was posted to Facebook.
All three Reform councillors joined the party last year.
Lidl said in a statement: “While our store openings are often attended by local officials in their capacity as elected community representatives, we are a non-partisan business and do not affiliate with any political party.
“On this occasion, no formal invitations were issued, and there was no official ribbon-cutting at our Cwmbran store opening.”
It confirmed it had contacted the councillor “to request amendments for accuracy in relation to their social media posts and in order to raise concerns from colleagues who had not consented to the external use of their images”.
The supermarket said that the photographs were taken “spontaneously after the store had already opened to customers”.