Port Raid Mission - only three of six players joined battle

[Michael96]

Hello.

Last week on Thursday me and five other Prussian players on the PvE Server formed a battle group and started a Port Raid mission at San Juan.

All of us met outside port and were next to our group leader, who then started the mission, but for some reason only three of us ended up in the battle - the group leader, me and another group member.

https://i.imgur.com/pwdSM0d.png 
https://i.imgur.com/MUWUNsh.png

The other group member that joined (Captain Miras), lost his built Santisima and I lost the purple L'Ocean that was given away as redeemable a while ago. Our group leader was able to get away from the enemies until the battle time was over.

Is it possible that this was a bug or are there any mechanics or modifiers we didn't know about that could have prevented the others from joining? 

I would appreciate if somebody could clarify the matter.
Thank you.