Cannot reproduce it no longer, when I spawned this thread...it was kind of constant... 
Actually managed to reproduce it:
And it happens, when you copy-paste regex (for example from regex101 or VSCode):
If you look at that highlighted area, this is what pasting adds, not sure what it is but it does look like huge amount of additional blanks.
When you type regex in, it won't happen.
If you delete that highlighted emptyness, everything works too (no wonder, because that regex is not valid)
It does not show Replacement after -> because it is pushed out of the window viewport.
Maybe adding trim to field contents would avoid such things?
Glad you found the reason for your observation! Thanks for taking the time to investigate further.
Yes, definitely. Unfortunately, I'm pretty sure that someone eventually doesn't want the field contents to be trimmed, but instead wants to have the additional spaces preserved as part of a regular expression or replacement string.
Right...well now at least know, what to look for, if it happens. 