Its just 12s13p. The load is equally(ish) distributed across all the cells.
Any potential imbalance by heavy load may drain the weaker battery faster but the voltage would equalise reaaonably quickly through the parallel connection.
IMO there’s nothing wrong here. If the board ran well off just the original 12s8p pack, its gonna be just fiiiiiine with the aux in parallel.
There is a lot to consider when paralleling packs like this, but unless you’re combining weak discharge cells with high discharge cells then chances are there’s a lot of stuff you don’t need to worry about.
Good discussion here Here’s what can happen when you parallel (mix) different cells in a pack
@Subbaz this is an epic first build dude! Congrats!