Indeed. Roam’s indentation isn’t copacetic with Markdown’s indentation for code syntax, and Markdown is rather picky about arbitrary indentation.
You may want to mass find-and-replace your tabs with more appropriate structures. I believe I did something similar when I imported some Roam notes a long time ago. Use an app with mass find and replace (e.g., VSCode) that recognizes Regex, and e.g., replace all sets of three tabs (\t\t\t, I think; try testing on regexr.com first) with e.g., \t\t- , then \t\t with \t- , then \t with - , etc.
YMMV—be sure to experiment with a subset of notes and make backups before you do this kind of massive editing.
As for Roam importing, feel free to create a feature request to add a “Convert tabs” to the importer plugin.