Strange fleet behavior while breaking a blockade

[Gorofin]

While I am breaking a blockade I attacked a single ship of the blockading fleet with my 1st fleet. I didn't declare a war. The fleet is in "Engage system targets" stance. My complete fleet destroyed the enemy target ship, all normal enemy ships attack me, ok. But they attack my lead ship of fleet 1 only (and the fighters at my starbase ?). They continue attacking my lead ship. But after destroying the enemy main target my fleet do nothing. They stay there and do nothing, only my lead ship still fights and loses...

I load this file up, but I started the game with 1.906 and play now with 1.907.