Why is my replacement battery charging to 96% then crawling to 100%

From a chemistry perspective, lithium batteries don't "like" to be empty nor full. Something like 20%-80% is the goldilocks zone for them. These days, a lot of devices prevent you from fully discharging or charging lithium batteries to prolong their life, which I suppose is why battery capacities listed by manufacturers can be oddly specific numbers in mAh.

I don't have an iPhone, but in your case it could be that this Optimized Battery Charging feature could be functioning exactly as intended. Normally phones take longer to go from 90% to 100% than from, say, 40% to 50%, but generally by an order of minutes rather than hours. It could be that when you have this feature on, iOS just really doesn't want the phone to charge to 100%. I wouldn't rule out an issue just yet, but that 96% - 100% charge isn't really worth waiting around for, bc it will drop below that rather quickly when you're using the phone anyway.