Мне удалось выяснить проблему.У меня не было ключевых кадров прозрачности и видимости в медиа-запросе (дух!).
Обновлен код анимации:
<amp-animation id="showSignup" layout="nodisplay">
<script type="application/json">
{
"duration": "200ms",
"fill": "both",
"iterations": "1",
"easing": "ease-out",
"delay": "200ms",
"direction": "alternate",
"selector": "#newsletter-slidein",
"switch": [
{
"media": "(min-width: 480px)",
"keyframes": { "opacity": "1", "visibility": "visible", "transform": "translateX(-365px)" }
},
{
"keyframes": { "opacity": "1", "visibility": "visible" }
}
]
}
</script>
</amp-animation>
<amp-animation id="hideSignup" layout="nodisplay">
<script type="application/json">
{
"duration": "200ms",
"fill": "both",
"iterations": "1",
"direction": "alternate",
"easing": "ease-in",
"delay": "0",
"selector": "#newsletter-slidein",
"switch": [
{
"media": "(min-width: 480px)",
"keyframes": { "opacity": "0", "visibility": "hidden", "transform": "translateX(-15px)" }
},
{
"keyframes": { "opacity": "0", "visibility": "hidden" }
}
]
}
</script>
</amp-animation>