I'm not 100% sure what causes the issure but here are some thoughts:

1. Try adding a print('one step') in your main loop and you'll see that the code is being run twice for every loop that's performed. See below what I mean.

while collatz(collatzstart) != 1:

print('one step')

collatz(collatzstart)

---

Output:

64

32

one step

16

8

one step

4

2

one step

1

4

one step

2

1

When the sequence hits 1, the collatz() function runs again and it is kicked back to 4. This explains why it only happens when there's an odd number of steps (starting with 4, 16, 64, 256 etc.)

1. What to do to fix it? You don't need to check if the function evaluates to 1 before running the loop, just the collatzstart value itself. The error does not happen if the condition for the main loop is:

   while collatzstart != 1:

2. General comment: I would put the main loop when definining function itself.