The Parliament of the UK has an appalling record on human rights in Northern Ireland, where the ECHR has long been the authority to which cases are appealed. Withdrawing from the ECHR both de facto and de jure weakens the human rights protections for residents of Northern Ireland.

Perfidious Albion cannot be trusted when it comes to Northern Ireland, at least.

The UK Parliament ratified the Treaty? How is its sovereignty violated by something it itself agreed to?

There is literally a move to do that. They are openly campaigning to repeal the Human Right Act and replace it with something that strips away most protections that are currently available to people whose human rights are violated by government.

The main changes being proposed are to give the government of the day more power and less judicial oversight, not parliament. Most changes are about removing people's ability to take the government to court. So it is the government of the day that becomes more sovereign, not parliament.

The changes don't affect parliament at all.

UK parliament is and always was ultimately sovereign on these issues. That was already true, with or without the Human Rights Act, and even when the UK was in the EU. (Otherwise there would have been no need for the UK to create its own Human Rights Act.)

In fact it was the UK parliament that put this judicial oversight of government in place in the first place by passing the Human Rights Act. Fashions change, and the current lot in power want to take the oversight away again.