I honestly have no idea. It might be a question better asked over at XF, maybe someone has ran across this before translating phrases for other addons or in the core.
I am going to guess that the phrase problem is my fault. When I would work on fixes, additions, etc. I would bump the current version to a working version such as 2.0.18.1, 2.0.18.2, and so on. Then when it was ready for release I would bump it to 2.0.19 and release it. What may have happened (and it looks like it did) is I may have inadvertently bumped it to 2.1.18.x while work was progressing and when it came time to release it I would have manually rolled it back to 2.0.19 to correct the version number but neglected to think about the phrase versioning.
Fixed the version number on +/- 20 phrases that had the wrong version number associated with them. This would cause translated phrases to show as outdated.
Thank you for looking into the issue in a timely fashion! Updating the add-on to v2.0.26 removed the phrases in question from the list of outdated phrases.
Fixed: A bug where users could submit a form multiple times if they had a direct link to the form even if they exceeded the form limit. Fixed: An error, [E_NOTICE] Undefined offset: XX src/addons/Snog/Forms/Pub/Controller/Form.php:409