Server Action Set Value - Unexpected Behaviour

So I’m digging into larger server side actions for the first time, and have come up against some strange behaviour.

I have a code structure like this:

Set Value amount = value1
if condition A then
     Set Value amount = value2
if condition B then
     Set Value amount = value3

Each Set Value command is set up to Output.

When condition A and condition B are false, amount is value1 as expected.

However, when condition A or condition B are true, amount is null, instead of value2 or value3.

Am I only allowed to assign to a value once?