You heard wrong. Nobody actually knows if you can transmit after getting vaccinated or not. Thus far anyone who claims otherwise is a liar.
There is some evidence that some of the vaccines prevent transmissions, but we need more study before we can make a statistical determination.
We know why other vaccines have failed to stop transmission. The reasons those vaccines didn't stop transmission do not apply to the covid ones. However there is a lot we don't know about biology so until we have better evidence no expert will state that the vaccine stops transmission even though many believe it will.
There is some evidence that some of the vaccines prevent transmissions, but we need more study before we can make a statistical determination.
We know why other vaccines have failed to stop transmission. The reasons those vaccines didn't stop transmission do not apply to the covid ones. However there is a lot we don't know about biology so until we have better evidence no expert will state that the vaccine stops transmission even though many believe it will.