I noticed this behavior too ; I think it's about PCs joining a battle when they are afar ; you have like infinite amount of realtime to look and join battle, but as soon you enter enemy sight, your character roll for initiative and is added to combat. So as you didnt got the best initiative, your character is locked before he/she can take their turn by initiative order.
The odd thing is you can attack with ranged spell/weapon, but sometimes you enter combat BEFORE your attack is issued, sometimes AFTER (the damage triggering combat for your character).
THe main issue is when you're planning synchronized actions for multiple group members ; if ennemies got better initiative than one of your chars, they can totally move to prevent your second character (wich was supposed hit them as the same time as another and trigger combat), and ruin your carefully prepared ambush.
I'll like to see option to trigger combat with everyone at once, or consistently triggering "suprised" bonus turn for your ambush to succeed.
Actually timing feels very clunky, and it happens on a regular basis that YOUR team is being surprised, even though your first action was to engage combat with hidden characters, no enemy being able to see your team... very frustrating atm.