I'm surprised I hadn't seen this before but I guess there are lots of portraits that haven't been widely shared although given that it was the final portrait she sat for it does seem odd it hasn't been out there much at least to my knowledge.
"Artist Basia Hamilton had the honour of painting a portrait of Her Late Majesty Queen Elizabeth II at Windsor Castle on May 6, 2022, the only private portrait sitting for the Platinum Jubilee year, and the final portrait HLM sat for."