In a documented workplace process, an apology is not a gesture of professionalism. It is evidence. Here is what conciliatory language documents, why it reads differently in the file than it did in the room, and what to say instead.
Being indispensable did not protect you. Here is why it never does.
When the absence of the workplace produces relief, something in the workplace changed, usually the Visibility Shift, happening on the days you are not there to see it.
When your communication becomes conditional on someone else's emotional state, the environment already shifted, and the Escalation Pattern is usually already active.