I think the arduino can't receive data when interrupts are disabled
So will the sender wait until serial can receive bytes again
or are the packets lost?
I'm using a leonardo with FTDI connected to pin 0 and 1.
Keep in mind that serial data reception (and sending) is not the only thing impacted by disabling interrupts. The clock can't tick, either. That is, millis() doesn't increment.
If have a timing critical sketch, where i need to disable the interrupts sometimes to do timing critical stuff.