I have the same problem when pasting code. I use windows-7, putty, some dev board based on STM32F103vet6, espruino_1v39_hystm32_24_ve.bin.
It seems it interprets each new character on-fly and if a string is long enough it loses some character. I think so because it loses same character each time. Pasting code in small chunks helps.
You can try to reproduce the issue using the code from my screenshot: