The battery in cause was not old, it was a new replacement battery, therefore 'old' battery issues will not apply here.

The problem is the replacement battery is not original, most surely you bought it from Ebay/Aliexpress and it looks like it's original but when you pay $5 one thing is sure: it's an aftermarket non original product. For this reason, the Lumia phone doesn't recognize it as original and for safety reasons, the phone it's programmed to not charge the battery more than 80%. Some aftermarket batteries won't have this problem, while other will.

There is absolutely nothing you can do to charge the battery more than 80%. The internet is full of people who have no clue but suggest tons of things, resetting the phone, updating the software, draining the battery, etc. etc. Nothing will help. DO NOT fully discharge you battery listening to internet 'experts', fully draining the battery is the best way to ruin it.