Is it normal to see focus inconsistency frame-to-frame with an f/1.4 lens?
It is normal to see slight AF inconsistency from frame-to-frame with every AF lens ever made. More precisely, it is normal to see at least slight AF inconsistency from frame-to-frame for every cameras/lens combination ever made.
The million dollar question is: How much frame-to-frame inconsistency should I expect? How much is acceptable?
The first, covered more below, can be measured and some generalizations made.
The second all depends upon the intended use of the camera/lens system, the size at which images will be displayed, and the budget one is willing to spend to reduce the amount of variability. Like many things, it costs a lot more to get from, say, 90% to 95% than it does to get from 80% to 90%. Getting from 95% to 98% is even more costly, as is getting from 98% to 99%, or from 99% to 99.5%.
In general, imaging sensor based focusing systems do better in terms of shot-to-shot consistency than systems that use a dedicated sensor for AF. But even with imaging sensor based systems, there will be some variation based on the limits of the system's ability to measure and the limits of the lens' smallest increment that it is capable of moving. You may also have slight movement of the focusing part of the lens that may occur between focusing and taking the picture due to things such as mechanical slack and gravity or other forces due to camera movement.
There are some tradeoffs to using imaging sensor based AF methods. By using much larger "pixels", dedicated PDAF sensors can often be more sensitive in lower light than CDAF or hybrid AF systems based on the camera's imaging sensor. They can often be faster because there is much less information that needs to be processed from a dedicated PDAF sensor than an imaging sensor.
Roger Cicala, the founder and chief lens guru at lensrentals, did a fascinating blog series several years ago. In it he compared several different camera bodies all using the same 24mm f/1.4 lens wide open with the center AF point. He also tested a few other lenses with varying introduction dates. He discovered that different camera bodies had varying amounts of standard deviation with the same lens. He also discovered that the better, newer bodies, which did better with newer lenses, did no better than the older cameras with older lenses. His conclusion was that it took both the newest bodies and the newest lenses that both supported a new "semi-closed loop" AF protocol to get the advantage from either the camera or the lens.
Roger Cicala's Autofocus Reality series is very insightful.
And: How Auto Focus (Often) Works
Also: Are zooms always sharper at one end than the other?
There are also a few questions here at Photography SE that you might find informative:
How to get more pin sharp shots with a lens known for inaccurate autofocus?
Should Canon 5D mk II autofocus be accurate enough for a f/1.2 lens?
Do all telephoto zoom lenses backfocus on the wide angle end?
Why isn't my Canon 70D autofocus accurate in manual zone AF mode with a 50mm f/1.8 lens?
Autofocus points in Mirrorless Cameras
Do the issues with sharpness I am seeing require AF fine-tuning?