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When trying to replace strings with parenthesis (in the given example - alternative writing of the words in Ukrainian) the sed replace performs correctly replacing all occurrences, while python3 re.sub() function replaces only first two

Argument 4 of re.sub is the maximum number of replacements, NOT flags:
Help on function sub in module re:
sub(pattern, repl, string, count=0, flags=0)
 Return the string obtained by replacing the leftmost
 non-overlapping occurrences of the pattern in string by the
 replacement repl. repl can be either a string or a callable;
 if a string, backslash escapes in it are processed. If it is
 a callable, it's passed the match object and must return
 a replacement string to be used.
re.I is 2, so you're telling it to perform at most 2 replacements.

See #11957.

Ahh, I guess I should have read the docs. But then looks like I am not the fist or only one so it's not as embarrassing. :)
I would support the proposal in #11957 to make re flags non-int object