there's the 3dB attenuation, but a strong signal is not enough in itself. Think of circular as diagonal - a mix of H and V. and from the perspective of circular signals, think of linear also as a mix of L and R.
So when you have a strong transponder that is circular, by using a linear LNBF, you get some of the signal, but not all of it, hence the 3dB attenuation.
However, let's say that was a Right-hand circular. Now let's change it to Left hand circular. The result is similar, you get part of the signal, with attenuation. If it's strong, it still works fine.
Now let's look at a crowded satellite, where most/all transponders are in use. They are usually alternating L/R, but in terms of frequency alone, they overlap. It works fine with the proper LNBF as it filters out the opposite polarity. But back to the example I discussed above, let's say there's a L signal and a R signal, with some overlap in frequency spectrum used by the transponders. The linear LNBF will pickup signal from BOTH trsnponders, and will mix them up, resulting in a mess and no reception.
So sometimes we are able to pick up circular signals with a linear LNBF and some thimes it just won't work... It will work only if there's no overlapping signal in the opposite polarity