Storing a new secret can cause more than one old secret to expire.

This depends on the (unspecified) order in which new secrets are stored.
For example, if the secrets for periods 0 - 2 are loaded in period 3,
the secrets for periods 3 and 4 will be stored; the secrets for periods
0 and 1 may expire separately if the secret for period 3 is stored
before the secret for period 4, or together if the secret for period 4
is stored before the secret for period 3.