PCMA within 4k TPC QPSK Upstream

Dear,
I would ask you if there is anybody who has experience with using PCMA with 4k TPC QPSK Upstream in iDirect 5IF HUB (XLC-11, X5, iNFINITI sw iDX 3.1.0.1)?

We have testing it and see that it starts to work from sym. rate 750kSps (QPSK FEC 0.793 4k TPC). If we use lower sym. rate it does not work anymore. The Downstream is 2650kSps QPSK FEC 0.793

Thank you for a help and any ideas.