Issue:
Web TimeSheet generates monthly timesheet for the users even though users' timesheet period type is defined as bi-weekly.

Cause:
This would happen if the new biweekly timesheet period created overlaps with the previous timesheet so that system cannot generate new biweekly timesheet on the start day defined in the new bi-weekly timesheet period

Solution:
In order to generate new biweekly timesheet with the start day specified in the timesheet period, it generates irregular timesheet for the current period to pick up the biweekly timesheet defined for users from next cycle.

If you want Biweekly timesheet to be generated for the current period, then there are only 2 options.

Delete Monthly timesheet generated, from Historical timesheets page for all the users.
Create Manual timesheet with start day as one day after your old timesheet period ends and end day as just one day prior to the day specified in the new bi-weekly timesheet period
Create Biweekly timesheet starting from the day you want the timesheet to be generated.

OR

Delete Monthly timesheet generated, from Historical timesheets page for all the users.
Create Biweekly timesheet with the start day as one day after your old timesheet ends.