Annoyingly all the ECGs I have done, even with hard, max heart rate efforts have come back completely clear.
I had atrial fibrillation and a couple of 24h ECGs came back normal, it's only because one time my GP heard something momentarily off that they believed me at all. On a 72h one it finally showed up and they took interest. I had a similar diagnosis of "it isn't affecting your quality of life, so just keep an eye on it" - until a couple years later my dad's similar condition worsened such that he could no longer have an ablation and had to have a pacemaker fitted - at that point they decided I should have an ablation. (None of this is covid-related though - this was several years ago)