@Bryan Santos Thanks for reaching out. I am unable to reproduce the issue inhouse am able to save the tag successfully in my basicv2 tier as shown below
You can also use this rest api https://learn.microsoft.com/en-us/rest/api/apimanagement/api-management-service/update?view=rest-apimanagement-2022-08-01&tabs=HTTP#code-try-0 to add the tags with below sample request body
{
"tags": {
"Owner": "sasolank",
"UID": "4f5025fe-0669-4e2e-8320-5199466e5eb3"
}
}
could you please retry and let me know if the issue persists