Hi Grnoem,
Thanks for your feedback.
When type 1, 2, 3, 4 and drop down the bottom right corner, you can select "Fill Series" to achieve 1, 2, 3, 4, 5, 6, 7, 8.
If you select "Copy Cells" or press Ctrl then drop down it, you will get 1, 2, 3, 4, 1, 2, 3, 4.
In addition, when you do the autofill, it is set default to fill series, here is an article for your reference: Fill data automatically in worksheet cells.
When I test to drop down the bottom right corner from my side, I get 1, 2, 3, 4, 5, 6, 7, 8 instead of 1, 2, 3, 4, 1, 2, 3, 4.
So I would like to confirm your Office product name, full version number and channel with you.
Please refer to this article and let me know the information like Office 365 ProPlus Version 1807 (Build 10325.20082) Monthly Channel.
Best Regards,
Gloria