Oh, no doubt you bear responsibility for your game. But that doesn't mean that getting around a nearly unhittable character with some workaround doesn't break your game. You couldn't, for instance, have an encounter with giants that was meaningful without adding servitors with spells. Or a dragon, for that matter, who cannot take advantage of melee attacks after breathing (absent legendary actions, of course, but those are very limited). You've essentially ruled out most of the Monster Manual as effective without adding something to specifically counter the cloak. Depending on playstyle, this can have huge repercussions in game. If I'm running a sandbox, where the world doesn't adapt to the PCs without cause as a matter of convention, then this is an issue not easily solved. Or, if I've already established some fictions about enemies (who they are, races, capabilities, etc.) then making sudden changes to that to "fix" the cloak is also not easily done.
And, thank you, you've again led me to the conclusion that "fixing" mechanical issues within the game narrative is a bad choice. If the cloak was an issue, it should have been dealt with at the meta level of discussion with players as to why it's affecting gameplay and then adjusting the cloak's mechanics or swapping it for something else. Any time you change the gameplay to offset something in the rules, that's a failure to deal with root causes, which quite often results in knock-on effects that are equally destabilizing.