how to work in hirarchical git model?

[Not sure I'm posting to the right mailing list; sorry in advance]

I’m working in a small company with about 15 developers. So far we were all 
rebasing our local repos on a single main repo. When one had some new commits, 
he rebased them on the main-repo/master and asked the main-repo maintainer to 
pull them in. So far so good, but we now considering moving into a three level 
model, where we’ll have sub-maintainers that will handle local merges before 
they hit the main-repo. I’m not sure I understand how this is supposed to work 
and I’d be thankful to get some advice. 

Here is one problem I saw when trying to work in the three-level model.
At some point, I had the following setup:

top-level : A----B----C----D
                  \
                   \
mid-level1:         K----L----M
                          \
                           \
low-level1:                 X----Y

The maintainer of mid-level1 has decided that commits K L M are ready to be 
merged into the top-level repo. So he rebased on top-level before asking 'please 
pull', but after that the low-level was not able to rebase on the mid-level 
any more. 
I can understand that the problem is because commit L had become L', and now 
low-level1 cannot find L anymore, i.e. now it looks like this:

top-level : A----B----C----D
                            \
                             \
mid-level1:                   K'----L'----M'
                          \
                           \
low-level1:                 X----Y

... but I guess this is not how it should work. So what is the right working 
flow for us?

Thanks.