Hi Sean,
There's nothing wrong with your P6201.Two recommendations:
1. Put the 2465B input at 1 MOhm DC or 50 Ohm DC, *not AC*. Use the P6201's External Terminator setting if the latter and in both cases use the probe's setting to select AC or DC coupling.
2. Compensate the P6201 at either 20us, 2 us or 100 ns / div, *not 1 ms* / div
Raymond
Raymond Domp Frank