The battery clip doesn't work well with larger batteries. There is simply not enough contact patch between the clip and battery connector pins to handle the extra current of the larger batteries. You can verify this after flight by measuring external batteries voltage and comparing to Mavic battery. You'll see that you've only used a portion of the external battery capacity. It gets worse as you use larger capacity batteries. Dual 1500 is about the ideal size for battery clip mod. For larger batteries you'll need to wire directly to main battery leads under Mavic top shell.
I am not completely agree with you.
Always, after the flight and later before recharging, I do the necessary check.
On the screen in DJI GO 4, I read the voltage and the theoretical % of the group, the external unit/s and the Mavic battery, both connected with unpowered motors.
Also after the disconnection of the added unit/s, read the two data on the app of the Mavic battery.
Later, at home, before recharging them, almost always I verify again the voltage of both, the DJI and external units, and I have always found comparable energy levels (read V).
Commonly with my missions, they are from low, to a very low levels: 20 ... 10%, or less, about 3.7 - 3.6 V, little less or little more, in total when still are both connected and motors disarmed and so is also when the batteries are separated.
Maybe that my 2 clips are made better, they are handcrafted, and significative variance is possible also for contact capability and Ohm resistence between contacts, there are visible differences in the form of the holes.
I have 2 units of these clips and I'll probably make one by myself, a different version for my needs and preferences.
Can say the same for my Inspire 1 on which I usually add two 6S LiPos 2650, 2700, 3000 mAh, vertical mounted on the left and right sides of the body, between the DJI TB48 battery and the 2 gear arms, connected with a clip (different clip, better for the contact for sure) at the main DJI battery, for a total of 11000 ... 11700 mAh, or if you prefer, more than 11 A.
The added weight to Inspire 1 Pro is 750 ... 820 g for these 2 batteries and few dozen grams for cables, plate and velcro.
Very similar voltages after the flight for each of the 3 units.
Only two or three times I have found that after the disconnection of the external batteries, the main DJI battery was switched off, zero LED, so the Mavic probably was able to fly at the end, thanks to the residual energy of the added packs.