By way of explanation only - all Items are initially created NULL. A Group would stay that way until a member updates, and then the aggregation function (SUM) runs. Of course other members might still be NULL etc.and that aggregation fail, for now. The resulting state would be UNDEF.
Writing that out is a useful reminder that group NULL state carries a significance - “I’ve never even tried to update” - different from UNDEF - “I messed up”.
In general, this item+group stuff is all reliable, barring config mistakes like mixing contact types into a number group and messing up any maths.
I would agree with your analysis that this is about a general system logjam, or more specifically an openHAB event bus logjam. The group updating stuff is likely just not getting a look-in.
I think Rich’s general advice here still holds
I do not expect there to be a magic solution, “fix that rule”. We suspect your system is built on an approach that exposes it to problems.