I noticed this happening with my reserves. The way around it is when you've been asked what players are available for the reserves, / U18's match go into the respective squad and select the team in the positions you want. If you don't have a full team but have e.g. two left backs, play the left back you want in position. The other will fill in one of the empty spots.
The reserve matches can be a useful tool for many of reasons: out of form 1st teamers, players coming back from injury, players needing some match fitness, useful youth players, players on trial etc.