Old Faithful is somewhat the exception to the rule here. It seems the petals can take water standing in the bud/open flower for a very, very long time before becoming infected with botrytis. I’ve never had any problems with flowers from them, notwithstanding the fact that the bud has a ‘rose-bud’ which is exposed to rain, cold and wind for a very long time. So it’s indeed not necessarily that kiss of death. But other varieties are not that resistant. I’ve given up on Moonrise as a cutflower because of these open buds, I got too many complaints from florists that the flowers opened with rotten stamens. I could leave the stems with open buds on the plants of course, not all stems do have an ugly bud, but the proportion of open buds in this cultivar has always been very high the years that I have been growing it. Too bad, as it does have many qualities, floriferous, early, large flower, large buds, yellow color, very sturdy stems. Although there’s also a second reason for not growing it anymore: it smells awkward, and I mean really, really awkward (less of a problem when growing it outside, but as a cutflower…).
You have noticed in some of your seedlings that they didn’t show those open buds when young, but at a more mature stage, they somehow appeared. Here the problem occurs one year more than the other. I’m not sure it has to do with the plants being more mature, the one location where Vanilla Schnapps showed the open buds, happened to be newly replanted divisions, whilst the locations with no ‘damage’ were older plants (2 years old and 3 years old). Therefore I would think temperature is more an issue. And also the fact that the sidebuds on these same young plants had perfectly closed buds, would make me think it’s the cold that somehow causes this. These sidebuds have developed somewhat later and they may thus have been in another less sensitive stage when the cold (wind?) damaged the further developed main buds. I’ll add an image of another variety Great Northern, which grew here for the first time, it also had an open main bud, but you can also see the sidebud perfectly formed. This is thus also a young plant with both a good and bad bud on the same stem. The open bud turned out to be a flower with several rotten petals and stamens in it, we’ll see if it is a returning issue. I don’t know what other factors could be causing this? Humidity? Soil? Soil probably not I’d think or the problem would be present each year and probably on every flower.
I’ve not noticed any difference between single, semi-double, or double cultivars. Some cultivars seem more susceptible than others. I should probably take notes what cultivars have most issues with it. And maybe ‘rose-buds’ are another issue, separate from ‘open buds’, the latter primarily being the fact that the outer petals are shorter than they ought to be.