These steps can be easily reproduced in a North America Cronus company.
1. Search for and open the Accounting Periods list page.
2. Insert a row between October and November in year 2019--or whatever year you choose.
3. Supply these values to the new row:
a. Starting Date: 15 October (of your selected year)
b. Name: Octember (yes, my values are fictional, but many companies operate a year with 13 periods that consist of four weeks each. if you prefer, then set up a year in your Accounting Periods following this ideology)
c. Count to verify that there are now 13 periods within the year in question
4. Close the Accounting Periods list page.
5. Search for and open the G/L Budgets list page.
6. Click +New to add a new record. Mine is for the year 2019 and has these values:
a. Name: 2019
b. Description: 2019 Budget
7. Click Process - Edit Budget to open the Budget page.
8. Ensure that the General fast tab has these values:
a. Budget Name: 2019 (or your 6.a. value)
b. Show as Lines: G/L Account
c. Show as Columns: Period
d. View by: Accounting Period
9. Review the periods in the column labels.

Expected Results:
There should be 13 periods displayed.

Actual Results:
There are 12 periods displayed. The last period which begins on 1 December is not included.

Workaround:
In the General fast tab of the Budget page, change the Show as Lines field to Period and the Show as Columns field to G/L Account.