I don't think the lowpassfilter is a matter if the LNB sychronises or not.
As DH2VA already wrote, the PLL synchronizes best with a high (but not to high) slew rate.
But the signallevel matters in some LNBs a lot.
Usually the x-tal oscillates with a level of around 0.5-1.0 Vss.
If you drive it externally with more than 1.0 Vss, some LNBs don't like it.
Then the DC-Level on the OSC out pin drops very low and the AC curve gets limited at ground or a frequency doubeling occurs.
You can easyly test that by inserting a potmeter (around 100 Ohm) as a voltage divider in the REF line.