I really want to stress that we should add the ticket for adding the missing test cases in *this* sprint and not postpone it any further.

-- "Isn't there something more important to be added instead?"

There. ALWAYS. Is. Something. MORE. Important. The real problem was that we implement the test cases in the past to begin violating our definition of done. We have to fix and one point and we have to own that decision as nobody else will care about passing tests and test coverage. It's our job to care for that.

Yes, we can instead focus on all the other high-priorities task that should have been done yesterday, yet that won't change the fact that large part our codebase will remain an untested messy blackbox just asking for weird bugs and wild goosechases in the future.

Don't hide behind "high priority tasks". A job is done when it is fucking done and tests are part of that. Hurrying from one important task to the next will just mean we'll never do it. There is no better time than right now.

If code coverage got left behind in the past, then we'll have to suck it up in order to fix it as soon as possible, otherwise we'll just suck forever.