Do you still have any of the reports open? I might have some debug info. This appears to be an error with consecutive months with 31 days.

Jan 2024 - Nov 2023 example (both cases show "2 months 1 day"):

https://www.wolframalpha.com/input?i=2024-01-31+to+2023-11-2...

https://www.wolframalpha.com/input?i=2024-01-30+to+2023-11-2...

Aug 2024 - Jun 2024 example (same bug):

https://www.wolframalpha.com/input?i=2024-08-31+to+2024-06-2...

https://www.wolframalpha.com/input?i=2024-08-30+to+2024-06-2...

The same thing doesn't happen from Jul 2024 - May 2024 (results vary by 1 day):

https://www.wolframalpha.com/input?i=2024-07-31+to+2024-05-2...

https://www.wolframalpha.com/input?i=2024-07-30+to+2024-05-2...