Historically I’ve found the AGC in the RTLSDR to be fairly poor - tending towards too much gain resulting in lots of IMD products and a reduction in available dynamic range. It’s also designed to work with signals much wider in bandwidth that those we are looking at.
The vast majority of stations in the SatNOGS network are going to be limited by local noise. In these cases, the gain should be fixed to a point where the observed noise floor on the SDR starts to rise. This maximises the available dynamic range of the SDR, and optimises the sensitivity.
I’ve written a guide on how to determine this point here: https://wiki.satnogs.org/Omnidirectional_Station_How_To#Setting_the_gain
For stations with very low noise floors, AGC may work OK (by virtue of it pegging the gain to full), but for many stations it is likely to cause more problems.
What is your antenna set up, and what sat were you looking at?