Seems like ambiskeletons require specific features that no one living creature have, but maybe some extinct creatures could had had them, the placoderms, a clade of extinct fishes in which almost the half of their body was covered by bone plates and for what I know, the soft tissue over the plates was minimum, thing which is important for the purpose of the "ambiskeleton, which is to permit big and massive arthropod like creatures with an apparently hard exoskeletal body.
As I mentioned these creatures had very little integument on the bone pectoral and skull plates (at laest just a thin skin layer at specific parts), which would easily reveal their tough appearance. Other thing is that they had most of the organs and muscles inside the boen plates and not out or over.
Other possible point to this "model" is that placoderms even underwater could reach bigger sizes than biggest arhtropods, with one of the biggest species having moderated estimations till to 6-7 meter long and the upper estimations with even to 10-11 meters.
So to had those big sizes is a possible evidence or solution respect to the problem that size is limited by having all its organs contained within a rigid exoskeleton which I've seen is usually mentioned for giant arthropods. Basically looks placoderms could grow without be limited by its armor.
And some amazing thing, a specific group of placoderms went farther, developing arthropod like limbs from its pectoral fins.
The following image is from a little size genre but others were able to reach to 2-3 meters long.
And the part which contribute to the "ambi" part is that placoderms kept a spine contained inside their armor, only connecting with their braincase, practically being an only internal muscle anchor point.
Trouble points to consider
These were acuatic creatures, so the more probable case is that all extra weight provided by the armor were sustented by water, that could cause problems on land for be so heavy. So would be necesary to determine how thick it could be before reaching a limit where it falls under its own weight.
Possible solutions for this are to use strange materials, or the evolving some estructural construction able to support the weight. Like this diabolical beetle or the mantis shrimp exoskeleton.
And things that I'm really not sure of how they work, like the biological mechanism by which these animals could grow within their armor without shedding as arthropods would, I have assumed that by a stacking of layers of bone dissolving into the lower ones as they grew.
And the mechanic part, I don't know if the surface provided together by the minimum internal skeleton and exoskeleton would be enough to anchor a sufficient amount of muscle to be able to move on land. So is that, to find how much surface muscles require.
The only thing we can know is that this "model" worked satisfactorily underwater.
By my part I would try to search for solutions or make questions to solve the last incovenients.