Boost converter voltage drop when charging battery
Looks like you'r boost converter is succsessfully charging the battery pack at whatever current it can provide and battery pack itself can absorb that current with very little of a voltage increase. LFP (LiFePO4) batteries do have VERY flat charge (and also discharge) curve beetween 20% SOC and 90% SOC.
I do suggest using ammeter to confirm charging and verify charging current is suitable for you.