The behavior you describe is normal and experienced by most people.
4.18 across the board is a very common voltage. It means that your cells are starting their normal degradation process. Most XRs probably spend most of their lifetime in this state, as the 4.20 quickly drops to 4.18. I bought my XR used with like 200mi on it and all cells listed 4.18. Like 2,000mi in and they still all list 4.18 after a full charge.
You probably just need to balance them a little better, leave the board plugged in overnight for a few nights in a row and you will likely be able to make it below 20% without issue again.
Be aware that the drop-off for normal cell degradation is not symmetrical, and it mostly happens on the bottom end on NMC batteries. There will be a time where your XR dies at 10% every time and you need to plan to stop riding before getting down to 20%.
Many people treat 20% as 0% on an XR and only ever plan to ride it down that far, with anything after 20% as their buffer in case of a miscalculation in how much they will use on a ride.