Can't Boot on Battery Alone

I have tried two iFixit replacement batteries for my refurb laptop.

The original HP battery is functional, but full charge lasts only for about 2.5hrs, so decided to replace.

After intalling the new batteries and fully charging them, the batteries last for 5+ hours, great.

However, if I try to boot the laptop on battery only, the keyboard lights up and power led illuminates for a couple of seconds before powering back off. The screen does not come on.

If I plug in the AC power, it will power on. If I let it boot into windows and remove the AC power, the laptop will remain working on the new batteries. If I boot to BIOS setup and then remove the AC power, the laptop will remain working on the new batteries.

Windows battery reports show the batteries are healthy. Windows HP diags show they're healthy. BIOS/UEFI diags show the batteries are healthy.

I've tried the various power resets, such as holding power buttons with/without battery connected. No luck.

I've reset the BIOS, no luck.

If I put the old battery back in, it boots up successfully without AC power.

Does anybody have any ideas?