Maximum resolution for A5000 less than 2560x1440?

Have a Dell 27" 2560x1440 monitor. With my A5000 and Quadro drivers I cannot reach this resolution in display settings.
No issues if drive my display from my older Titan Volta installed on the same PC.
Could this have something to do with using a Display Port to HDMI adpater?

It can very well be. What monitor model is that specifically? Maybe the datasheet of the Monitor states if HDMI input even supports more than 1080p. And not all HDMI adapters support more than 1080p either.

We are talking about DisplayPort out from the GPU to HDMI in on the Dell, correct? The other way round (HDMI to DP) you would need an active adapter in any case.