I know there are tons of threads on the inaccuracies of race predictor. I've had so many runs well below (like 20' below the marathon predictor) and many other training runs well well below the race prediction that I don't think it's even adapting anymore.
Also included here my Vo2 max has increased in the training period to 66 (down from a high a few years ago of 69) but while the Vo2 is down, it's up in the training cycle from 62 to 66. Again I don't think Garmin 's software understands the idea of "max" , however even if we consider the max to instead be a vdot, an increase should see faster predictors. Instead my race predictors have actually gotten a little slower as this value has gone up.
I understand the device doesn't always get it right. I don't even think I'm an outlier, however we should be able to enter a recent race time as a guidepost to have RP start again near those times. It's one thing to be off by 10-30 seconds, its another to be off by 15-20 minutes, even after multiple marathons.