I don't think that's the case at all really, I suspect you're reading too much into extremely small sample sizes. Bruno doesn't consistently under-perform his xA in a way you would expect him to if what you described was the case. In fact last season was the only time he's under-performed in that metric over an entire season since he got here. Likewise in the last 3 seasons before this one Eriksen has underperformed his in a way he wouldn't have if what you're describing is true.
I think the simple reality is that a few of Eriksen's assists have been the result of a seriously good bit of forward play (Garnacho vs Fulham, Rashford's cutback from the penalty kick, the Casemiro finish from the freekick vs Forest) which have rewarded some good deliveries. Those deliveries aren't rewarded equally or evenly and Eriksen will probably make similar quality deliveries that get missed later on in the season which will even things out a bit.
Compare the quality of
Rashford's back post header for the Eriksen assist vs
his miss vs Brighton from a Fernandes pass, or even
his like for like 3rd goal vs Arsenal. He's also had some other bad misses from his passes
1,
2 (
found the video, 59 mins here),
3,
4 (40 seconds in).
When you're talking about incidents which occur in single figure numbers like assists this early in the season you're going to have your data badly swung by outliers.