Example
<s>
Strikethrough
Strikethrough in editor preview only.</s>
No strikethrough.
Try
<s>Strikethrough
[quote="jooize"]
Strikethrough in editor preview only.
[/quote]
Strikethrough in editor preview only.</s>
No strikethrough.
<s>
Strikethrough
Strikethrough in editor preview only.</s>
No strikethrough.
<s>Strikethrough
[quote="jooize"]
Strikethrough in editor preview only.
[/quote]
Strikethrough in editor preview only.</s>
No strikethrough.
Your problem is multi line here.
The editor preview shouldn’t suggest that the text in the submitted post will have strikethrough across lines, assuming that’s the way it’s intended to work. In any case there’s currently an inconsistency.
Duplicate of Multi-Line Strikethrough Not Covering Quotes though this came first, the other topic is better documented.