The meta_query parameter always need to be an array of array ans should be written like this

"meta_query" => array(
     array(
         "key" => "_fixtures",
         "value" => $fixguid, 
         "compare" => "="
     )

In fact, the array of array is use to serve to any relation when you want to query multiple metas, but it's need even you query only one.

Hope it works after !

The meta_query parameter always need to be an array of array and should be written like this

"meta_query" => array(
     array(
         "key" => "_fixtures",
         "value" => $fixguid, 
         "compare" => "="
     )

In fact, the array of array is use to serve to any relation when you want to query multiple metas, but it's need even you query only one.

Hope it works after !