Does crossing multiple sub-stat upgrade threshold at the same time force every bonus to be applied on the same sub-stat?

When upgrading artifact pieces in Genshin Impact it is possible with some luck/planning to increase the level of an artifact by more than 8 levels at a time. Every 4 levels an artifact gets a boost to a random sub-stat OR a new sub-stat if it currently has less than 4. This means that an artifact that is boosted at least 8 levels from a single upgrade should apply two sub-stat upgrades at the same time. This in turn brings us to a question...

What happens when multiple sub-stat upgrades are applied at the same time? I think that new stats take precedence, but what about an artifact that already reached 4 sub-stats? The few times I saw this happening to players it looked like every increase was applied to the same stat instead that randomly for each upgrade as expected.

Basically it looks like increasing a 4 sub stat artifact all the way to level 8 should roll for a sub stat upgrade twice separately, but when level 4 and level 8 are reached at the same time what really happens is that the system rolls ONCE for a single stat to upgrade and then twice for each upgrade to apply.

Is this true? I doubt this can be abused anyway but it would still be a nice trivia to know.