Okay, sarcasm detector going off here, waiving a towel in front of it, bloody thing it is still beeping, batteries don't need to be changed, but I'll bite anyway.
The problem is currently how Testing is used.
Unstable contantly has new packages dribbling into it when they become available. This includes upstream Arch Stable packages and Manjaro managed packages like kernels, drivers and tools. All good this is what unstable is for.
Testing is currently treated almost like a secondary unstable repo, minus the development. Packages are promoted to Testing in small batches, quite often. Sometimes there is an announcement thread if something significant is promoted (ie DE update, python update, etc), but most times there is not. It is up to the testing user to keep on top of these changes and test accordingly.
When it comes time to promote these packages to stable, many of these small testing batch updates are combined into a single large batch update. This large batch update process has not been tested, and this has been the source of ALL the major Manjaro update issues since I've been using it.
@c00ter has also pointed this out in this thread, multiple times.
@anon35400795 has been banging on about this for a while, and I agree with him now, there needs to be a 1:1 mapping between batch updates applied to Testing and batch updates applied to Stable.
A more robust change management procedure is required where the exact same large batch updates that are applied in Stable get applied in Testing first.
This would have helped enormously during this libglvnd update, and another very problematic update it would have helped was the notorious systemd upgrade nightmare. In that instance there were no issues in Testing because systemd was incrementally updated from 2.31-1 to 2.31-4, but when this change was applied in a single update in Stable it crashed systems during the update and left many systems in an unrecoverable state.
Had this batch update been applied in Testing first, the issue would have been caught earlier and remedied before migration to Stable. Same with this update and the libglvnd GLX issues.
Problem is I don't know how much of an appetite there is for this type of change from within the Manjaro Team, given it would involve a little more change mangement overhead and a slightly more restrictive use of the Testing environment.