I can use title commands, and they execute, but only actionbar commands will actually appear.

This can be recreated by having either a player or a command block use any title command other than "/title [player] actionbar" (because that works). Input any string, colored or not, and-at least for me--it will not appear.

I have been experiencing this bug since 1.6.X

I have found, however, that if I play in an imported world, that title commands from command blocks work in those. But in worlds I create, only actionbar titles work.

CONCLUSION: In worlds that I create, title titles and subtitle titles do not appear, but they do execute.