When strikethrough is used with multiple lines, it works if the lines that are inside it don’t have empty lines, otherwise it might make some lines without strikethrough.
The main problem here is that the preview shows as if all lines will receive the formating, but the actual post won’t.
Example:
Strikethrough line 02.
Strikethrough line 03.
In the preview, Strikethrough line 02 was with strikethrough.
Raw:
<s>
Strikethrough line 01.
Strikethrough line 02.
Strikethrough line 03.
</s>
Related:
The above topic is related and was solved for multiline, but it seems it doesn’t work when there are empty lines.
The ideal scenario is for all lines to have strikethrough. If this is not feasible, I would at least expect that the composer be consistent with what is shown in the post (be it with all paragraphs in the middle formatted or not, whichever is easier to implement).