Core WordPress will not retain text formatting in blog, archive and search lists. This can be an issue if you are using long or full content excerpts read from the main text, rather than the excerpt panel. Most themes have enhanced the correlated templates to preserve formatting. However, some themes have not.
Once choice is to customize the templates with PHP coding.
A quicker solution is to use the Advanced Excerpt plugin. Although it has not been updated in 2 years, it has over 100,000 active installation and is created by a reputable organization Delicious Brains. Delicious Brains produces advanced plugins including WP Migrate DB Pro and WP Offload S3.
Below is a screen shot of the various settings. Important parameters include number of excerpt words or characters, selective disable for specific archive templates and termination options.
