On that toilet, that vent you installed is a flat vent. The first time that clogs the vent will seal off from waste buildup that has no way to self cleanse and clear back out. Just random flushing in that line can also cause residual flow back into that arm, building up till it clogs off.
Constantly see this on the older homes, regardless of piping choice.
That's why they disallow that type of venting now.
Would of been better to snake that 3" over to the wall, or 45 degree angle 3 X 1.5" and roll the vent up, then go where it needs to go.