There is a bug due to Windows that limits lists to about 1600 items.
Do you have some version of Windows?
Does the ENTIRE list of collections have more than 1600 items?
Using Windows 7.
Not entirely sure how long the list is (how can I check?) but on average there are probably more than 10 collections per collection set so I guess the answer is yes, it does have more than 1600 collections.
Is there a way to fix the bug? Does it exist in subsequent versions of LR?
The bug is in Windows, not Lightroom. It doesn't matter what version of Lightroom you have, the bug will be there until Microsoft fixes it.
The usual way to get around this bug is to collapse portions of the list, so that the only portions of the list open are the specific collection set of interest. If this can't be done in your case, then I guess you could try re-arranging your collections sets temporarily as a test to see if you can get it to work properly.
The bug is in Windows, not Lightroom.
Or in Adobe's failure to write their code to handle the Windows limitation.
Fair enough, but it sure doesn't seem to me like Adobe is going to fix this, so I believe this bug will be present in Lightroom until Microsoft fixes it.
The maximize size of a window at the lowest software layer in Windows is limited to a maximum height of 2^15-1 pixels (32K pixels). Most software that needs to present long, scrolling lists to the user repaints the window with the current view into the list. For example, the LR Library grid view does that as you scroll thumbnails (logically, the grid view can be much, much larger than 32K pixels high). Higher-level user-interface libraries typically handle all of this repainting for the application developer; but it appears that for some user-interface controls in Windows LR, the Adobe developers neglected to do that (perhaps because Windows LR doesn't use any of the common higher-level libraries).
The 32K limit has been in place since at least Windows 3. It's not at all uncommon for window systems to have such limits at the lowest levels, relying on higher levels of the software to provide infinite scrolling views. So I think it's very unlikely Microsoft will ever change it, at least not until we have 32K displays :->