It feels exactly that way to me. It's as of every kiss they have is strictly on impulse but that's it. We don't see Eustaph holding back his desire or Lan feeling flustered for a long time but this might have to do with the fact they have known each other for a long time. Like maybe they've loved each other for so long that loving as lovers isn't much different from them loving to the point of sacrificing each other's life for each other.
I know it's the translation, but at chapter 102 (recent chap) when Eustaph called Lan "Sis", it kinda gave me the ick. It's hella weird, but considering how that's what he's been calling her for long time in the past, it might have been natural for him. However, if they are lovers, I'd rather they not mention anything about being raised as siblings or whatnot.
I feel like the romantic moments are out of nowhere and they seem isolated. Like there's no lingering of feelings or sexual/romantic passion bleeding into their interactions. Like no heated gazes being crossed, or desire to keep doing things even when the plot is moving. Like they just cut their feelings off too cleanly between scenes.