When you do a 'replace' the last occurence of the pattern found will be updated with the new pattern. The bug is that it keeps thinking that the last occurence is a different string with the pattern. Hence you could keep hitting on 'replace' and it will keep appending to the same string.

Steps to reproduce the bug(silk 5.32)

1)select in menu Edit|Replace
2)input 2 strings to find and replace
3)keep hitting on 'replace' only

if you hit other buttons they work fine and the 'replace' button also works afterwards.