Search bar, `multiSelect:false` for `field_value_selection` tosses all values when trying to select a new one

[andrew-moldovan]

I've reproduced this on the documentation page. Go to src-docs/src/views/search_bar/search_bar.js and do multiSelect: false, for the tag field. In the UI, pick a tag see that it's correctly working. Then pick another tag and instead of replacing the original value with this new one, the component just tosses the old value and the new value, leaving you with nothing selected.

This is almost certainly a regression because we've been using this component the Cloud Deployments List page for years and it would just grab the new value instead of tossing both.

[cee-chen]

I can repro, and this was also reported in #6540. I'll try and prioritize a fix this week.