Add to queue vs. Play next

Hi guys,
I'm running PMS v1.1.4.2757 on NAS and I'm unable to add audio tacks to the end of the current queue, using the 'Add to queue' feature.
Instead, the new tracks are added as next in the queue and, subsequently, the previous ones are lost. Instead, I would expect this behaviour from the 'Play next' feature. This occurs both from desktop web client and the android client so, I believe it has to do with the server itself. Please let me know your thoughts.
Thanks
Marco