I have a Trek Domane SLR, and I assume the fork or at least steerer tube is the same. As I recall the manual for the bike shows the maximum amount of spacers on top of the stem and I think that happens to be all the spacers that come with the bike.

I asked one of the senior people at my local Trek store this same question and he said it’s not an issue. That the steerer does NOT rely on the plug for support from the stem clamping pressure. He also went on to say that this concern dates back to the early days of carbon use and it’s really not a thing for modern bikes. Of course, he can’t speak for the entire industry.

As for the contradictory advise, not everyone gets the memo and your first response may just someone restating the old wives tale rather than knowing what they are talking about. If your manual is like mine and lists the maximum spacers allowed on top of the stem, I’d trust that. Something in writing in the manual likely has thought behind it from people who really know. Not just some rando tech support guy that may have just started at Trek the same day they responded to your email.

While it’s not your job to educate the Trek tech, I would politely ask him to double check the issue as his advise is contradicted by the manual, if it is. Which it seems you may have already done, but got a reply from a different person. If so, nothing else you can do.

I ran my stem with a full set of spacers above the stem. So quite the chimney. I did this for at least 6 months until I was confident in the position before having it cut down. I still have a little space above, 10 mm or so. I’ve had no trouble with steerer tube.

I thinks it’s obvious given the short plug, that Trek does not expect the plug to support the steerer tube against clamping pressure.