Toilet sink drainage query

Hello,

I just had my toilet sink drainage pipe replaced today because the old metal one was leaking and when I tried to tighten it the whole pipe snapped off (the new one is also a metal one with a larger lower trap body). I noticed a problem after the replacement: Whenever I turn the tap on at the highest water flow, water started raising and leaking out of the overflow eye on the sink. I did a few experiments and noticed:

1. If the flow of water is low or medium, water does not raise up in the overflow and no water comes out from it.
2. If I put a bucket under the tap and run the water at maximum flow (so no water gets into the drainage), no water comes out from the overflow.

The new drainage pipe was installed by someone who is not an expert in plumbing, and he said he had no idea why water is coming back out of the overflow (he also didn’t do a very good job on the pipe itself, as there are a couple of leaks at the joint points). I have used the sink for nearly 3 years, this had never previously happened with the old drainage pipe. The person who installed it is trying to say I have a blockage in there and his installation of the drainage pipe has nothing to do with the water leaking out of the overflow, but this never used to happen before the installation. Please would you be able to give me advice on why this is happening? And how can this be fixed?

Thank you very much for your help.

Kind Regards,
Jack