Trim level and colour? It would be interesting if the issue were limited to colors/trim lol.
The ST has had a few of those type of issues but none has affected me. Primarily they had a batch of bad transmissions early, they had a recall for a wiring problem, and an issue with the fuel evap or something. There were a couple of eye roller recalls as well, one of the halo lights was white but required to be amber by federal law, and the hatch would release at up to 8mph instead of the federally mandated 5mph or something like that. Several people have reported a false "headlamp fault" issue, which I myself have seen about 4 times in 6 years, but it seems to be a benign thing that clears up when the engine is restarted. My point here is that I wouldn't let a very sporadic thing like that persuade you from making the purchase, though 30 members from one 30K subreddit (of which maybe 1/3 have 2019s??) seems like a fairly high rate.
From reading those threads, it seems that color actually makes sense--not because of the color but because they were built in batch, by color. The first reports all seemed to be the Urano Grey Rabbits, and those happened to be the first 1k of that trim that were built, followed by cornflower blue, et al. So, if faulty part or other issue with the first cars on the line, it would be those Grey Rabbits.
However, it seems to have been reported across all colors and trim levels, and different ranges of manufacture dates. Seems a primary culprit is that the vehicles are stuck in Transport mode, and/or a faulty camshaft alignment. Seems the DSG has also been affected. A lot of users have blamed themselves, because many are going back to MT after a long absence, which is understandable. I would have/will do the same thing.
So, it seems like a fairly easy fix and from those user reports, the dealerships have been accommodating to the problem. My only concern would be that one guy that stalled at highway speeds. ....around here, that means I've suddenly been pancaked. (fortunately, a lot of the highway traffic is actually stop and go, though, haha).
It hasn't discouraged me, only thing being I'd hate to pick up a car then spend the first 1 or 2 weeks diagnosing a problem in the dealership. It is something I will bring up though--"please check transport mode!"
....but at least I'd still have my current car. State Farm tells me that I would have up to a month to keep it on the same policy before selling it, without any increase in cost (basically new cost would be to insure the GTI, but since the Mazda is on the same policy, I get to carry over the year/accident benefit on that policy without having to reset it by starting a new policy for the new car)