@Tomáš Ludvík Apologies for inconvenience on this, Acknowledged the issue is reproducible and product team is working towards fix, will update here accordingly.
Workaround suggested.
Use Azure CLI command to generate a token:
az staticwebapp hostname set -n MyStaticAppName --hostname example.com --validation-method "dns-txt-token"
Once executed, a TXT token will be generated. But the command will not complete until the validation complete.
You can retrieve the token via Azure portal > Static Web App > Custom Domain.
You can also retrieve the token with below Azure CLI command in a different command prompt:
az staticwebapp hostname show -n MyStaticAppName -g MyResourceGroup --hostname example.com --query "validationToken"
Please let us know if further query.