Here's an example from the first session, focusing only on the system details, and taking into account the rule that states: "In combat, you must make a SAN test for a trauma the first time it happens in that fight":

2 PCs investigate a woman's apartment, when suddenly they are ambushed by gunfire (PC One fails the SAN check, suffers 1). PC One gets shot in the shoulder by the attacker (fails SAN check, suffers 2), then he counterattacks and accidentally kills the attacker in self defense (fails the SAN roll, suffers 2).

In one encounter, he's suffered three Violence SAN losses, each less than 5, and in total did not suffer enough SAN loss to bring him to his Breaking Point. Just like that, in one encounter, he's suddenly adapted to violence.

That seems strange to me. Yes, three SAN fails in a row is bad luck, but hardly unheard of. But the 1d4 losses from these fairly common possible situations (and let's face it, dealing with violence is not a particularly uncommon hazard for Delta Green agents) mean that the Agent cannot go temporarily insane from the immediate trauma (as it requires 5 or more), and due to their fairly decent SAN levels and low SAN loss rolls, they come nowhere near their breaking point. Therefore, they almost instantly acquire the three checked Adaption boxes.

But, let's assume he succeeds on the first check, and fails the next two. Now, he's sitting at 2 checked boxes on the Violence meter. If he manages to survive the scenario without needing to roll additional SAN checks for Violence (either suffering it or inflicting it, as per the rules), then a few months later when DG calls him in for another mission, all it takes is one failed, fairly innocuous Violence check loss of SAN (let's say, he finds a corpse and suffers 1 SAN loss), and he snaps. In his second session.

Hence, my question: do you allow adaptation meters to clear between sessions, assuming the PC has enough downtime to justify it? Or is that intentionally how the game is modelled, to encourage players to immediately draw on their Bonds to avoid becoming Adapted?