Did you activitate this "Critical Update" in your org?
Behavior Change for Default Values in Flow Screen Fields
There are two ways that a flow screen field can be visited multiple
times.
The same instance: The user who’s running the flow enters information
into a screen field, clicks Previous and then clicks Next.
Different instance: A screen is used within a loop, and the flow
requires the fields to be completed independently each time.
Previously, flows used the last-entered value for a field on a screen
if the screen had been visited before, regardless of whether it was
the same instance of that screen.
If you activate this update now, flows use the default value (if you
set one) for the field when visited for the first time for that
instance. If the user reaches the screen because the flow is
navigating through a loop, the default value for that field is used.
If the user reaches the screen by navigating back&em;or by navigating
back and then forward&em;the last-entered value for that field is
used.
How can I modify my flows to use a previously entered value? To use a
previously entered value as the default value for a screen field:
Within the loop, assign the value of the screen field to a variable.
Set the default value for the screen field to that variable.