Help needed configuring battery bank out of smaller cells
Uneven charging is the reason. The risk you haven't addressed is the possibility of improperly charging one of the batteries when they are connected in parallel. In particular, you have no way to guarantee that the charging current to each battery is below its specified maximum current. If one battery is older, or has been discharged further, then it may draw more than an equal share of the charging current. Worse, if one cell in one battery fails shorted then that battery will draw the lion's share of the current which may cause overcharging and the release of hazardous amounts of hydrogen gas.

One way to prevent overcurrent is to limit the total charging current to no more than the maximum charging current for just one battery. Unfortunately, this will result in slow recharge times, particularly as the number of batteries increases.