Burnout is real. It can happen to any team size. It is for a leader to keep an eye on the team. Apart from a great working environment, transparency, a leader has to be aware of setting up an honest expectation.
A leader has to nudge, encourage and empower every single team member. The radical belief in the organization bigger cause keeps a great team together.
Your engineer will not tell you about being in burn out, you have to be with them enough to realize it.