For example the following f-string

f'{1):.1%}'

IDLE will scroll to the top of the file and highlight a random and irrelevant line (not sure what it's doing tbh).

running the expression with `exec` makes it look like implicit parenthesis are added to the ends of files..

>>> exec("f'{1):.1f}'")
Traceback (most recent call last):
  File "<pyshell#2860>", line 1, in <module>
    exec("f'{1):.1f}'")
  File "<fstring>", line 1
    (1))
       ^
SyntaxError: unexpected EOF while parsing

IDLE can correctly find a `(1))` in a file but appears to fail only in f-string expressions.

This is not an IDLE bug, per se. There's a known problem with f-strings not producing the correct line numbers. See issue #29051. Unless Terry thinks there's something IDLE-specific here, I suggest closing this as a duplicate.

Syntax errors come from Python, not IDLE.  Except for the pseudofile name ('<pyshell*nnnn>' versus '<stdin>'), the message is the same as the interactive interpreter.  Anytime one thinks IDLE has a bug, one should check the standard interactive interpreter as a 'control'.