I managed to solve this on my own. I am leaving the solution here in case anyone else runs into the exact same issue.
The cause:
The theme developers did not update the main translation template (xts-theme.pot) with the new strings from the Waitlist form (located in woocommerce/single-product/wtl-form-not-signed.php). Because of this, Loco Translate simply doesn’t see these phrases during a standard sync of your language file.
How to fix it without custom PHP snippets:
Go to Loco Translate > Themes > Antares.
Do not open your specific language file yet. Instead, look at the top under the Overview tab and click Edit template next to the xts-theme.pot file.
Click the Sync button inside the template editor. This forces Loco Translate to scan all the theme’s PHP source files and pull the missing strings into the base dictionary.
Click Save.
Now, go back to your specific language file and click Sync there.
After doing this, all the missing Waitlist strings (“This product is currently sold out.”, “Add to waitlist”, etc.) will finally appear on the list and can be translated normally.
.
