Exactly what happened to me in Portugal. The visa policy was the cheapest thing that met the requirement, and I didn't read it until I needed a specialist. What I check now, in this order: outpatient cover, existing conditions, the list of hospitals it works with, and whether I have to pay first and claim later. The last one matters a lot if something big happens.
Visa for Thailand required proof of health insurance. I bought a policy that ticked the visa's boxes. Only later did I read it properly.
It covers hospital stays. Outpatient visits: barely. Existing conditions: no. Evacuation: no. So it satisfies the visa but wouldn't cover most of what I'm likely to actually need.
Question: did anyone else buy the minimum for the visa and then add proper cover? Or go straight to a full international policy? What did you check that you'd tell others to check?