I thought that this had been fixed in firmware 5.90 or 6.20. Do you know what version you're running on your watch? Beware though that these updates brought new bugs including freezes after saving an activity, so stick with 4.20 if stability is more important to you than this feature.
I came here now looking for feedback on Race an Activity. It looks like I need to test it out myself in training before using it in a race. (Though of course that's the old adage, nothing new on race day!)
I'm disappointed with Garmin. I bought the Forerunner 645 Music just because of the function "Race an activity". Unfortunately, after a few trainings, I have to say that this feature is useless. Although I'm going slower than the reference activity, the watch usually shows ahead time difference. In addition, time difference is changing oddly, e.g. from 2 sec. jumps for 1 minute despite knowing that the reference training was smooth and there is no reason for such jump differences. I used the device in bike mode, firmware is 4.20 and I'm really afraid to upgrade to 6.20 due to freezing problems. I do not know what to do :(
So I risked it and upgraded to 6.20 firmware. The watch has not frozen yet, but problems with the feature "race an activity" have remained. Almost the entire workout watch shows that I'm going faster although I know I'm going slower. E.g. watch shows that I am 1 minute ahead durig all workout and about 1 mile before finish the difference suddenly changes to 1 minute behind (which is right). Used in bike mode. I will try it in run mode. I'm unhappy, this is very imortant feature for my trainings.
Are you still having issues with this feature? Yesterday I had a problem with my Garmin 945 racing a previous activity of 8,75 km, that was wrongly estimated in 6 hours by Garmin. This is the description of the issue https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-945/260877/bug-on-distance-racing-a-previous-activity , but it is very similar to some comments of some people in this thread. It is very strange the issue is here after 3 years...
This is happening on my 6X Pro Solar.
For me the ETA seems good but the behind / in front is stuck at a set time - 38:50 from the start and doesn't move off this.
Not very handy, and I use this a lot for training... Disappointed
I found a workaround that works for me. Check here forums.garmin.com/.../how-to-race-an-activity-hint-don-t-race-an-activity