It's a problem downstream. Integrators weren't forced to include a version number for v1, so the rework overhead to use v2 will be higher than if it was present in your scheme to begin.
^/api(/(?!v[0-9]).)?$ is v1
^/api/v2(/.)?$ is v2
It's really not an issue in any case, it just itches your brain, because is not as neat as you would like it to be.