EPIPHYLLUM x "LETTY MAY"
The plant in question had even produced a bicoloured stem. This confirms the assumption of the contamination by a mosaic (phytovirus) whose activity would be quite present in cold periods and absent in warm ones. The virus seems to be thermosensitive.
Since then, I learned that to get the strawberry plants rid of the marbling virus, they had to be cultivated for some time at temperatures slightly lower than 40°C. This goes in the direction of the variations observed on “Letty May”.