as highlighted by Shah et al  in the article “Misunderstood parameter of NCBI BLAST im- pacts the correctness of bioinformatics workflows”,
the -max_target_seqs parameter doesn’t necessarily behave as intended in the public BLAST. Some discussion of this is here  and can also be found elsewhere.
This is a problem in BLAST so unfortunately there is nothing we can do to fix it in Sequenceserver. We hope that NCBI/BLAST+ developers will soon address this. In the meantime we recommend not using that parameter.