Thanks, All, for the responses. I am now quite sure that I dislodged the spring whilst cleaning. User error. And another learning experience.
I was able to reseat the spring using a tiny crochet hook. I'll be more careful going forward and will check the lifter each range visit.
Hopefully, the spring does not need replacement. Disassembling the decocker sear cage is as much fun as a toothache.