This is another interesting point you raise, John!
The concentration cycles are tied to the CISSP cycle, and for my ISSAP and ISSEP the endorsement approval email even stated the cycle start dates as being the same as for my current CISSP cycle.
Based on that, theoretically you could retrospectively apply CPEs to the concentrations which were earned for activities which were undertaken prior to you having even been awarded your concentration(s), and I'm sure the CPE portal would not prevent this.
However, I think the ISC2 Code of Ethics should be considered at this point, and, as such, I won't be applying any CPEs retrospectively.
Additionally, for my ISSMP, which was my most recently earned concentration, the endorsement approval email stated the cycle start date as being the date I was awarded the ISSMP, so theoretically this would remove the possibility to apply CPEs retrospectively. I don't believe there's any difference between the ISSMP and ISSAP or ISSEP in relation to cycle dates, so the change in the email advising the cycle start dates just seems to be based on what I presume is updated guidance.