What's up with these plated through-holes / vias?
What's up?

First, I assume your board has two layers only (top and bottom, no internal layers).

It may be the case that there is not enough copper in the vias. Copper is applied to vias in a galvanic or electro-chemical process. Maybe the solution used in the process was old, maybe the boards were not put into the process long enough, or maybe the board was not prepared or cleanded properly before they were put into the process.

It also happens that during soldering, the board expands more than the vias can handle and the copper layer cracks (almost) open. This is especially tricky with boards containing a polyimide layer for rigid-flexible-combinations. After a while, the crack goes from almost open to open. These failures - like many other issues you can have with PCBs - are nasty, because they are hard to find while the boards are still tested. Instead, they occur after a while in the field...

My personal experience is that, however well your production tests are done, boards remain a case of trust between the manufacturer and the customer. Money spent on a good board manufacturer is money well spent.

Repair:

For home brew devices, this would be the way to go. For mass production, you would have to rework all vias, because any via could be weak and almost fail open. This is likely not a practical solution, and also very likely not a repair method recommended by IPC guidelines...