"Is Dewi Lingo ill today?"
Translation:Ydy Dewi Lingo'n sâl heddiw?
The mutation is required. Without seeing the question, your exact answer and the exact response by Duo, it is hard to say if there was a system problem or not. Using tost here instead of the correctly mutated dost is something that Duo's algorithm might flag as a mistake or simply as a typo.
If you do come across a system bug, please report it so that Duo staff can investigate - https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-