Backup retention interval is unstable

[Urth]

The interval between backups in the same retention period is unstable. For example when a daily backup is selected as the best candidate for a weekly retention period it does not consider the previous weekly backup. As a result the interval between weekly backups can vary between 6-8 days.

Select planb-20200622T0014Z as fresh match for 20200621T0013Z (planb.storage.base)
Select planb-20200621T0004Z as best match for 20200621T0013Z with diff:540 (planb.storage.base)
Select planb-20200614T0005Z as best match for 20200614T0004Z with diff:-60 (planb.storage.base)
Select planb-20200609T0012Z as best match for 20200607T0005Z with diff:-173220 (planb.storage.base)
Select planb-20200604T0012Z as best match for 20200602T0012Z with diff:-172800 (planb.storage.base)

Because planb-20200622T0014Z is the closest it will be preserved and cause planb-20200621T0004Z to be removed even though it is the perfect match compared to the previous backup planb-20200614T0005Z.