firstSpawnTime And Interval Behavior should be expected.

[gadget114514]

Current implementation:
Start -> first spawn time -> [spawn] -> firstspawntime -> [spawn] -> interval(fixedDelayBetweenSpawns) -> [spawn]

Expected implementation:
Start -> first spawn time -> [spawn] -> interval(fixedDelayBetweenSpawns) -> [spawn] -> interval(fixedDelayBetweenSpawns)

Fix code:
void SetupTimer() {
timeLeft = firstSpawnTime;
ElapsedBehaviour(); // set interval

        Elapsed += Spawn;
        Elapsed += ElapsedBehaviour;

#if UNITY_EDITOR
UltimateLog(string.Format("SetupTimer"));
#endif
}