It's probably coming directly from the stem, the graft that was used to create the standard. You can snap it off at the base.
From
https://www.rhs.org.uk/plants/...
"The rounded canopy of foliage and flowers on a Standard rose is grafted onto the stem of a species rose such as Rosa canina or Rosa multiflora. Any shoots, (also called suckers) that sprout further down the stem should be removed as these will be from the stem species. "
