Its from your Site Search 360 plugin’s CSS and JS loading late. It needs to be optimized.
If you check your site’s page source, the plugin’s CSS and script is the last one to load within the <head> tag.
The initial styling you see is indeed from the theme stylesheet when the plugin’s stylesheet has not loaded yet. But that’s only until the plugin’s stylesheet loads. Once plugin’s supposed stylesheet loads, it will apply.
This particular sequence happens within seconds. The change between the stylesheets used is what we perceive as Layout Shifts.
To address this, consider using an optimization plugin. Aggregating CSS (stylesheet merge) may actually fix this issue.
A wise man once said:
"Have you cleared your cache?"