That's quite interesting — I do not get that behaviour at all, both on newly installed and upgraded systems from older LTSes (3 laptops and a desktop). They are all on 18.04, and I do have at least 2 layouts on all of them (Serbian Cyrillic, Latin and sometimes US English) which I switch between hundreds of times a day.

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.

I just noticed that the above bug refers to switching input methods vs. just keyboard layouts.

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.