MT revert translated UI elements back to English #2605

Describe the bug
We've got a report from the French community that SUMO bot revert the already translated UI elements back to English in some articles:

• https://support.mozilla.org/fr/kb/marque-pages-firefox/compare?locale=fr&to=309488&from=291238
• https://support.mozilla.org/fr/kb/modifier-date-ou-heure-%C3%A9v%C3%A9nement/compare?locale=fr&to=309001&from=139780

Preconditions

Steps to reproduce
Steps to reproduce the behavior:

1. Go to SUMO Bot translation that has prior translation
2. Compare it to the previous approved translation

Expected behavior
Machine translation or SUMO Bot preserve the already translated UI elements in target language using the prior translation

Actual behavior
Machine translation or SUMO Bot revert UI elements back to English. Looks like it happen sporadically.

Screenshots/Screencasts