Below are both datasheets, it appears there is a soft turn on delay difference per page 7 of the datasheets.
RT6575
RT6576
Applicable Part Numbers
RT6576BGQWTR-ND
RT6576BGQWCT-ND
RT6576BGQWDKR-ND
RT6576BGQW
RT6576BGQWTR
RT6576AGQWTR-ND
RT6576AGQWCT-ND
RT6576AGQWDKR-ND
RT6576AGQW
RT6576AGQWTR