OK – so i can see now the AMP plugin is disabled.
The Column Width adjustment is adding Inline CSS – this is a function of the core columns block.
The AMP plugin strips the inline and moves it to the head of the document ( so this is just an after effect ).
Problem is Gutenberg developers didn’t consider how inline CSS would mess up any theme styles in regards to responsiveness. Just tested on 2019 and the same issue applies.
You can use this CSS to force the themes styles to apply:
@media (max-width: 769px) and (min-width: 600px) {
.wp-block-column {
flex-basis: calc(50% - 16px) !important;
flex-grow: 0;
flex-shrink: 1;
}
}
Problem it forces columns to 50%.
Personally i would not use the Column Width adjuster as they have royally made a mess of it and use CSS Classes on each column.
UPDATE: GitHub issue here:
https://github.com/WordPress/gutenberg/issues/18416