Richard Jackson wrote:
May we call it a bug?
. Is there something else we might call it?
. Is there something else we might call it? Hmm, its PTC - maybe its a feature!? PTC marketing sure can make that possible.
Has anybody an explanation why the symbolic solve produces that many wrong solution if used without the modifer "fully"?
Because there's a bug
Yes, and obviously "fully" makes the symbolics to think about its results a second time 
Alan's explanation make it clear whats happening - false solutions coming from taking the equation to the power of 4.