People who switch between several languages just can't use Ubuntu because of this, read some testimonials on the bug linked above.
This is just one of the pain points though. There were many.
Of the non-standard settings, I've got "make Caps lock an additional Ctrl" and "each window keeps its own layout" on. The rest, including keyboard switching shortcut (Meta+Space) is using the defaults.
I simply press the shortcut and start typing and works as expected — if I find the time, I'll debug further and report on the LP bug, but just wanted to report that I do not experience any of the problems mentioned.
Input methods are a separate concept from keyboard layouts (XKB), and I only ever use XKB stuff. Input methods load separate libraries which interpret keycodes, and are commonly generic enough to allow "external" complex input definitions (think Arabic or CJK) — not sure how fast they used to be before, but perhaps combining IM and layout selection is the culprit.
"Any Ubuntu 19.x" because non-LTS Ubuntu releases come out every six months, so there was Ubuntu 19.04 and Ubuntu 19.10, but never an "Ubuntu 19": they are never as polished or as stable as LTS releases, and are only supported for 9 months, forcing you to update regularly.
If you are looking for a more stable experience and you are not chasing the latest features, you would probably be better served with LTS releases which come out every 2 years (they have hardware updates with newer kernels included every once in a while, so you do not have to worry about using them on latest laptops either).
If you want the most stable route, go with LTS point (.1) release. Eg. I only update my servers when 18.04.1 or 20.04.1 is out.