Why were such large changes rushed into an LTS release?
My sentiments exactly.
We hit at least 6 or 7 hugely critical, production stop bugs just like this when doing our latest upgrade and we've had to devote significant time and effort just to get back to a state similar to what we were in before the upgrade - and that work is still ongoing today after weeks of effort.
I'd like to say this is an isolated circumstance but the last 2 or 3 upgrades we've tried since our adoption of the tool a couple of years ago have had similar results. In fact, I've even gone so far as to question whether there is any value whatsoever in adopting the LTS edition at all. It's benefit is supposed to provide a stable working environment for production use but in-so-far as upgrades are concerned, that seems far from the truth. Then, to make matters worse, fixes for the critical bugs are expected to be rolled out and tested on the mainline first, meaning that fixes take way longer to get released on the LTS branch - which compounds the problem.
Really, we love the tool - when it works. But managing upgrades is so painful that we may need to consider adopting a different tool in the longer term. We just can't afford to spend this amount of time and effort managing the tool, and the cost of the downtime it causes for our production teams.