I think the Prune Simulator is not showing how the pruning will actually work over time, but only if it was pruning your backups for the very first time.
For example, if I set it with a daily backup job and set keep-daily=14, keep-weekly=4 and keep-monthly=12 and an end-date of today (2025-02-21) it will show keeping monthly backups on the last day of each month. But it also shows deleting the daily backup on 2025-01-31, so it won't be there to become the monthly backup for January. Instead, the last weekly for January (2025-01-26) will end up becoming the monthly backup.
Because of this, the simulator should be showing all of the monthly backups being the last Sunday of the month rather than the last day of the month since this is what will end up being the case over time.
For example, if I set it with a daily backup job and set keep-daily=14, keep-weekly=4 and keep-monthly=12 and an end-date of today (2025-02-21) it will show keeping monthly backups on the last day of each month. But it also shows deleting the daily backup on 2025-01-31, so it won't be there to become the monthly backup for January. Instead, the last weekly for January (2025-01-26) will end up becoming the monthly backup.
Because of this, the simulator should be showing all of the monthly backups being the last Sunday of the month rather than the last day of the month since this is what will end up being the case over time.