Steps to reproduce (Windows/Python 3.7.7):

1. Define replacement string that starts with an integer:

REPLACEMENT = '12345'

2. Use re.sub() as follows:

re.sub(r'([a-z]+)', fr"\1{REPLACEMENT}", 'something')

3. The outcome is not 'something12345' as expected, but 'J345'.

Note that I am using the group in the replacement argument, which is a raw f-string.

A quick investigation with other replacement strings renders similar unexpected behavior:

REPLACEMENT = '1': leads to re.error (invalid group reference 11 at position 1)
REPLACEMENT = '13': 'K'
etc.

So it seems like the f-string is evaluated first, yielding a string starting with an integer. Python then interprets the '\1' to indicate group 1 as '\1<first integer>', which leads to the behavior described above. Even if this is by design, it seems confusing and makes using groups with re.sub() cumbersome if the replacement f-string starts with an integer.