It only happened on the "Sleep here" route which avoid MC's encounter with 
Nyxie and 
Irena.
1. 
Siera affection value is initially set as 4.
2. The code checks for 
Siera affection value is 6 or greater before playing 
Siera 'romance' scenes ( s_aff >= 6 ).
3. Maximum value for 
Siera affection value is 8 (as per 
v0.6b).
4. MC can gain 3 affection points if he forgives 
Siera during the astral projection scene to escape his captors.
5. MC can gain 1 affection point if he hugs 
Siera when they meet each other the first time at the Ninja Village.
The maximum affection points for MC (if he was not kidnapped) is 5 (because 4 will not happened). IMO it make sense that in order for the 'romance' scene to play out, the MC should have forgiven 
Siera which unfortunately only happened on the "Go home" route.
The "Sleep here" route is like implicitly declaring the MC doesn't want to forgive 
Siera yet.
Personally, I'd go home to 
Siera every single chance I get, even if it cost me my life but I don't know what would 
Zycris do instead

?
But anyway, it is up to 
Flynn974 how to reconcile the "Forgive 
Siera" scenario.