Pressure Cooker - Same Settings, but much tougher meat
I'm assuming since you said "this time... still a little frozen" that you were starting from frozen Thighs the times you had success, and that the only real difference is that these particular thighs were not fully thawed. If that's not the case you could have another issue entirely.

In these kinds of situations I think the best we can do is use the information available to try and guess at the cause. Rather than the issue being that the thighs were overcooked and thus tough, I think the issue was likely that the thighs were less cooked than the tender ones you're used to. If you're leaving the Instant Pot for the same amount of time, the internal temperature of the chicken thighs would not effect the overall temperature of the device much at all since it is insulated by the surrounding meat.

Starting from a cooler interior of the chicken thighs (frozen vs refrigerated) could mean that the thawed thighs were cooked past the threshold of toughness and into the point of being soft/tender again, whereas the frozen thighs wouldn't have yet reached that point due to the temperature difference.

This is, however, just a guess. The only thing you could really do if this happens again, to try and test this, would be to put the thighs back in and cook them for a proportionately longer time.

Best of luck!