The skill "Energy Transfer" in the "Natural Flyers" tree balances the shields between a player and his summoned fighters one every 0.25 seconds. I noticed that when using this skill, the shared shield seems to stall out at ~76% of the maximum capacity. Any shield beyond this percentage is lost / drained, as is any shield regen that would tick it over this percentage limit.
The attached savegame can be used as a quick test. The character starts with a full shield. Summoning a slot 1 minion will drain both the minions and character shield to 76%. Summoning more minions temporally increases the shield capacity available, yet after a few ticks all shields will level out around 76%.
A sidenote is that this only seems to trigger when the "Cheap Raider Bay V" in the first skill slot is summoned. Summoning the bombers in slot 2-4 does not trigger this issue at all. However, once a slot 1 minion is summoned, the "76% shield" seems to apply to the player plus all his minions.
Savegame and character
WeTransfer Download Link