Replaced battery fail after following instruction to let it shut off

I replaced a battery in an LG G7 ThinQ that wasn't holding charge well (age) and would causes spontaneous device shutdown when at lower charge levels and something would cause a power draw spike.

The replacement is the IF303-092-1;

LG G7 ThinQ Battery

After installing the battery, everything appeared to be fine. It came up as 60% charge, and I followed the instruction on the box to "Charge your newly installed battery up to 100% and keep charging it for at least 2 more hours. Then use your device [...]". Charging went fine, and after waiting the device powered up just fine.

The next instruction reads "Then use your device until it shuts off due to low battery." I set up a youtube playlist to play overnight. The phone was shut off in the morning.

However, plugging the charging cable back in only gave the vibration motor buzz and a 'grey' screen, with no indication of it being charged. I left it for several hours, but there was no change. Turning it off (only possible when unplugged) and back on again provides no change. Every once in a while I see what appears to be the beginning of the LG G7 ThinQ's charge indicator circle only for it to go back to a 'grey' screen.

( When I say 'grey' screen - it might well be black, with just the backlight bleed. )

Replacing the iFixit battery with the original battery again, the battery starts charging and the battery charge indicator appears as normal (just takes a very long time to charge).

Swapping it back out for the iFixit battery gets me back to only a 'grey' screen.

Did the instruction to let it drain cause the battery charge to be so low that the device is refusing to charge it?

I'm thinking of starting a replacement / refund procedure, but want to make sure I'm not missing some other step I could take.

( If anyone were to suggest factory reset: though I'd much prefer not to have to perform a factory reset regardless, Vol- + Power also does nothing with the replacement battery in place, whether on charger or not. Have not yet attempted with the original battery in place. )