You should definitely speak to your union. To be clear, you had one five-day absence which resulted in sickness monitoring, were then off with stress caused by work and they’re holding a capability meeting? That sounds like it’s escalated very quickly.
I am not a lawyer or in HR, just a manager, but a few thoughts.
Was the meeting while you were off sick just a keeping in touch meeting, or a formal sickness review? If it was the latter, check your workplace policy for if they should have sent you a copy of the meeting notes for you to agree or disagree. Also if it was the latter, did they inform you of your right to be accompanied by eg a union rep?
Your employer has a legal duty to protect you from preventable stress. You say they’ve been supportive, but have you been referred to occupational health and have you completed a stress risk assessment with your line manager? And did you have a formal return to work meeting? My HR would have kittens if I tried to hold a capability hearing without doing those basic things for someone who had been off with stress. And have you been informed of your right to be accompanied to this meeting? Read your policies very carefully and check if they’ve been followed.
I am struggling to understand why they are holding a capability meeting if there are no performance concerns and you’ve only had two periods of sickness.
As to the missing notes, I would be inclined to report that as a data breach - if someone does not know what they’ve done with notes about your health, which is special category data, that is not OK.
Please call your union asap.