Imagine two capacitors in series initially charged with a load.
Which cap decays faster? The smaller one.
Which cap charges up past full voltage faster? The smaller one
BMS units are intended to shunt current on discharge with current on the stronger battery so it appears as weak as the weakest cell in order to prevent the weakest cell from getting damaged from undervoltage damage. Then during charging shunt the weakest cell to bypass current from exceeding the CV max while the stronger batteries catch up.
Normally 6cell batteries are matched <0.1% when new and the CV phase allows the current to decline while the stronger cells try to catch-up in series.
What happens without a BMS with external mismatched packs is the weaker packs accelerate faster in aging in a runaway condition from under and over charge, so you never want battery packs mismatched for Ah or ESR for longest life cycles.
This also means choosing to use less CV and cutout voltage reduces capacity but total life cycles increase dramatically. (E.g. 10x for 50% capacity utilization re Battery university, your mileage may vary)
It also means unless you have dozens of cells , when a string or module has 1 dead cell and the others are near end of life, it’s just a short matter of time before the next cell is dead and then the next etc. so batteries are like Caps and must be balanced as they perform as well as the weakest link.
To prove the point here, I modelled your 50Ah batteries to show how each battery gradually changes with respect to the others. It uses a comparator to decide when to charge 10A and discharge 10A with a simple battery model initialized to 12.0V then counts each cycle displayed on LEDs in binary. The voltage is sampled every second and the run time shows after about 50 cycles the capacity difference from Vmax has shifted by a spread of about 0.5V out of 14V. Eventually when one pack exceeds absolute max or min, degradation accelerates. this proves the point that a master BMS is needed to track all packs and need to be charged separately to same voltage after 50 cycles.
Here is the simulation.