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The weirdest thing happens to date calculation because of what you see below; notice that adding one month to Oct 31st takes it to December! This can't be? I have the same issue on Ubuntu 16 and 20. This issue also causes my shell script to give the wrong date, so it's not just the cron scheduler.

root@ubuntugc:/home/miuser# date -d "20210930+1 month" +%Y-%m-%d 2021-10-30 root@ubuntugc:/home/miuser# date -d "20211030+1 month" +%Y-%m-%d 2021-11-30 root@ubuntugc:/home/miuser# date -d "20211031+1 month" +%Y-%m-%d 2021-12-01
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This is how date works, when adding 1 month it just increases month by one, in your case to 2021-11-31 which is a date that doesn't exist so it then moves the result one day forward to a valid date (2021-12-01). From coreutils documentation:

The fuzz in units can cause problems with relative items. For example, ‘2020-07-31 -1 month’ might evaluate to 2020-07-01, because 2020-06-31 is an invalid date. To determine the previous month more reliably, you can ask for the month before the 15th of the current month. For example:

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