I'm having the same problem with MEC not providing the ability to update to version 3.4.6. I'm using the latest version of Pro (which is validated) and everything else on the site is updated the latest version.
On the plugins page, the MEC plugin says, "There is a new version of Modern Events Calendar available. View version 3.4.6 details. Automatic update is unavailable for this plugin." The updates page doesn't show any updates as being available.
In the past, this behavior has seemed to appear when a plugin presents updates that aren't actually supposed to be available to us as Themeco customers. Is 3.4.6 of MEC the latest version that's available to us?