Any champ with delayed damage, will always function well as a tank
Therein lies the problem with Ekko. Almost all of Ekko damage is delayed. Naturally, if you can live long enough, you can access that damage. Now the thing is, AP Ekko damage is significantly high because he does not have easy access to his damage ratios, and that is why tank Ekko is so successful.
It fundamentally bypasses a core weakness of AP Ekko and allows tank Ekko to be practically broken with no inherent weaknesses. The problem is, AP Ekko is losing a significant part of his utility because of this and AP Ekko, needs that utility to access his delayed damage. Otherwise, he won't function very well as he will not be able to access his damage reliably enough.
IDK how riot is going to fix this, but AP Ekko should not be made to suffer for tank Ekko.