I don't know why whoever wrote the NuGet package decided to completely ignore your own API docs that say this field is a string, but this property's definition needs to be corrected. Just because it looks like a hexadecimal number, doesn't mean it's a hexadecimal number.
For anyone else struggling with this, because they need to use FeedResult.ToVersion as the fromVersion in a subsequent call to GetFeed, you need to do the following:
var fromVersion = feedResult?.ToVersion?.ToString("x16");
Of course, should this property be corrected to string, the above will no longer be necessary.