Summary: Initiative can change during combat by leveling up, and it is not in favour of the unit that leveled up.

1. I had a not-cat at the last position of the Initiative Order.
   
2. It picked up a supply crate, thereby earning enough exp to level up.
   
3. Its initiative, and therefore position in the Initiative Order rose accordingly.
   
4. The unit that went before the not-cat (in this case an enemy) was now the next in Initiative Order.
   
5. Effectively, the other unit got an extra turn.
   

Before:
36 - Notcat A
27 - Notcat B
21 - Notcat C
13 - Rubble Sprite A
13 - Rubble Sprite B
13 - Juvenile Corrupted Maple C
12 - Notcat D
(not sure what Notcat Ds initiative was)

After:
36 - Notcat A
27 - Notcat B
21 - Notcat C
14 - Notcat D
13 - Rubble Sprite A
13 - Rubble Sprite B
13 - Juvenile Corrupted Maple C


Expected behaviour:
Initiative Order does not change (in the enemy's favour) during an ongoing battle.

Collecting related threads! Squid replied in the first linked thread that this is going to take creativity to fix.

I can't believe I didn't find the last one, given it uses the same wording in the title. Sorry

Gonna lock this as a duplicate of the above