The teambuilder will automatically set a Pokemon's Attack IV stat to 0 if it carries Foul Play as its only physical attack. While this is optimal in most formats, it is not in either Gen 8 Anything Goes or National Dex AG, where Yveltal is one of the most ubiquitous Pokemon and situations where one may want to Dynamax it are very common. I imagine there are more examples in other formats with Dynamax, but that's beyond my knowledge.
This is related to other issues with the teambuilder's IV suggestions (see here, here and here), but not much has been done as far as I know. At the least, it would be much appreciated if this were to be changed in both Gen 8 AG formats, as I don't believe a fix would be hard to implement (please correct me if I'm wrong!).