You push the tag, CI goes green, the package lands on PyPI. By evening, nothing.
Not ingratitude and not burnout. While the goal stood, it held the schedule: it was clear what to do on Saturday and what to think about while standing in a queue. The goal went, and structure went with it. Joy, if there was any, lasted forty minutes.
The usual explanation is that it must have been the wrong project. Or that you are bad at being happy. Both are wrong and both are convenient, because they offer something to fix.
The mechanism is simpler. Achievement does not produce meaning, it spends it. The meaning was put into the work in advance, over months, in small amounts, and at the moment of completion it is all written off at once. The emptiness afterwards is not a fault. It is the normal balance after a withdrawal.
You can check this against the previous release. What did you feel a week after it? If nothing, then it is not about this particular project.
Hence the habit of starting the next thing the same evening. It works: a new goal restores the schedule instantly. It is also why you never stop, because stopping now means meeting that same emptiness without anaesthetic.
None of this means you should stop finishing things. It only means the day after a release is worth planning in advance — not as a reward, but as a place where there will be nothing.