Upon further investigation, this appears to be related only to users who have upgraded to a higher plan (our settings do not allow downgrading).
Settings1.png (attached graphic) shows upgrade workflow settings, which is identical on each of our plans.
Settings2.png (attached graphic) shows pricing and validity settings for one of our plans. The only setting here that changes from plan to plan is the Regular Price of plan.
It appears that somehow the setting that we had previously set for "Prorate remaining value of this plan when upgrading" somehow got set to "Yes," when it was ALWAYS set to "No." This means that an upgrade of CB Subs at some recent point actually changed this setting somehow. I thought I'd call this to your attention, as it screwed me up for awhile today, and probably will be doing the same to other CB Subs users who have or will upgrade in the future.