after 8 months of struggling with this issue, and 6 months since the bug has been officially recognized by adobe, I am still experiencing an issue with using "b" to choose images for quick collection in GRID VIEW missing approximately every other photo if done in anything other than a glacial pace. It appears if you quickly select a sequence of 10 images (for example) by using the 'b' key every other one adds and then IMMEDIATELY removes itsself from the quick collection (you can see it happen very fast in the prompt on the screen 'added to quick collection, then removed from quick collection)
it only happens in grid view, it has happened on new and old catalogs, several different computers, one of the bigger updates late last year initiated the bug for me, I've been using LR virtually every day since 2008.
One of the tech support people was able to reproduce it on his computer as well. Many others suggest I have a computer issue or resource issues (i had an i9 macbook pro, and now I have an m1max macbook pro as well as an m1 13" macbook pro, my colleagues have reproduced it on their computers as well ranging from mac minis to mac pros and even windows machines.
I am very much looking forward to a fix for this problem, as it effects how I interact with lightroom all of the time.
I know there are workarounds like selecting images by flagging them or color coding them, but I have been using B for quick collection my entire career and it's a shame this bug has been introduced and unresolved.