I feel your pain and have had the same sort of issues. I once spent 3 months on a hard to reproduce issue to prove it was the closed source software from a vendor that was at fault.
Bugs have definitely been present since the start.
However, since we moved away from waterfall (not that I really miss it), systems are not that carefully designed anymore. Even if at the start the system is well designed, careless additions start creeping in and before you know it there are bottlenecks and race conditions everywhere. I think this was caused by the "move fast and break things" and the reduction of QA teams.
In other words, if it works in my small and non-representative environment, ship it. If it breaks, someone will tell us and we will fix it. I don't like this either, but yeah, it's been like this for a while now.
Latest development is IMO moving even more in this same direction, so it's getting worse rather than getting better :(