Could be. I do have some significant Thinkpad experience (going back to the IBM days) and I do know that they will not alter the model number regardless of what’s in the machine, but you can pull a build sheet from their website with the serial number. Do you know if your HPs could have had this happen? Was your distributor HP or elsewhere? (Not hating, just curious!)
Capacity loss over time is a decent idea. The 2nd Gen machine has an 11th Gen chip, vs 10th Gen in the Gen 1, and quiet possibly is able to burn more power quicker as well. Thinkpad power consumption is also definable in the BIOS or Vantage software for many of them, so those settings all could vary.
Normally I’d be happy to help troubleshoot this sort of thing but frankly I’m not sure OP was looking to chat.