I test on 3.5 and 3.8 running not in an func and I don't have the problem:

Python 3.8.0a2+ (heads/bpo-36287:ba8f342623, Mar 25 2019, 21:57:16) 
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 'bar'
>>> b = 42
>>> c  = a + ' = c'
>>> c
'bar = c'
>>> exec(c)
>>> 'bar' in locals()
True
>>> print(locals()['bar'])
bar = c
>>> print(bar)
bar = c
>>> print("'bar' OK:", bar)
'bar' OK: bar = c