"Undrain" is not idiomatic, at least outside of Google. One might drain a tank or creek to empty it, the reverse isn't "undraining" to fill it back up. "issuance flow has been restored" might be a more widely understood phrasing.

Admittedly, a nitpick, however the tech industry has a tendency to invent new words when they could say the exact same thing in plain English and be better understood by a wider audience.