What a good question.
幹部/干部 is a very versatile word as you have discovered. It can change quite a bit depending on the context and company. So let's first look at the basics and expand outward.
One of the meanings of the character 干 is "the main part of something"
i.e. 树干 - tree trunk, 干道 - main road, 躯干 - torso.
部 in the context of work means 部属 - subordinate, soldier.
So let's put both of those back together - a "干部" is someone important and central to the department or business and they are also a subordinate or soldier.
Generally speaking, 干部s are people that hold central roles or have specific skills or responsibilities, yet they are not necessarily the leaders or the heads of the companies or departments.
That's why the term is often translated as manager, even though that might not be the case.
I think perhaps a more apt translation might be "key persons (to this project or department)" or "key members". It does, of course, depend on the particular structure of a company to give a more accurate translation.
If I heard "干部会议" I would think of the people that are central to that particular department or task, not necessarily a meeting of managers (although that might be the case depending on the size of the organisation, because the managers might be those key persons)
I've seen the term "储备干部／儲備幹部" since returning to Taiwan, which I guess would translate as "reserved key persons" or "Key persons in waiting" (lol). A lot of companies in Taiwan hire under that title, usually targeted at fresh graduates. These titles can also imply graduate training programs or training programmes for managerial roles. In big firms, I guess these programmes are more establish. In smaller companies, however, I feel advertising this way can be a little misleading because of the ambiguity of the word "干部”, let alone a reserved 干部.
Hope you found that useful and let me know if that clears up the term for you.