I'm able to record through my amp's aux-in jack: is this normal?

[zuppermati]

Hello everyone.

Around one week ago I wrote a post (you can find it here: Recording with a phone through the amp headset output: is it dangerous?) to ask you guys for some clarifications regarding my sketchy-but-functional setup to record, by using my phone recording app, my Fender Rumble 100's output through the "PHONE OUT" port, by using a bunch of 3.5mm Y-splitters and an aux cable.
Today I was doing some experiments out of pure boredom with my setup and I've casually came out with a totally unexpected result. Essentially:

I've plugged the male end of a 3,5mm jack Y splitter into the AUX IN port of my Fender Rumble 100 V3. One of the 2 female ends is kept free while the other one is plugged into one of the ends of a male to male 3.5mm aux cable.
The male end of the aux cable is then plugged to the microphone female port of another y splitter. The male end of such splitter is then plugged into my phone.

So basically my phone is plugged into the AUX IN port of my amp.
Now: as far as I'm aware, such port is only supposed to allow the amp to output whatever is playing from the device which is plugged into it.
Except that, for some reason, my phone can record the amp's output while plugged into it with my setup, which makes no sense since if I plug my headphones straight into the AUX IN port, i get no sound through it (which is normal I guess).
Another weird thing is that, while on this setup, if I play any track on my phone, it doesn't get played through my amp. However, if I plug another set of headphones into the other female end of the y splitter (the one plugged directly into my phone), I can hear the output normally.

So I wanted to ask you guys why is that and if this setup could damage in any way either my phone (which I doubt) or my amp.

Regards.

 

[Crater]

No, it won't harm anything. The "aux in" is simply the preamp's output signal that's run through a speaker simulator filter and then goes to a signal mixer to combine the aux in signal and the bass amp signal. In reality, the "signal mixer" is most likely just a couple of resistors, so the amp signal can "back feed" to the aux input jack. If you carefully examine the block diagram in the Rumble's manual, you can see why this works.

You could also use your amp's "FX send" as a direct out.

 

[zuppermati]

No, it won't harm anything. The "aux in" is simply the preamp's output signal that's run through a speaker simulator filter and then goes to a signal mixer to combine the aux in signal and the bass amp signal. In reality, the "signal mixer" is most likely just a couple of resistors, so the amp signal can "back feed" to the aux input jack. If you carefully examine the block diagram in the Rumble's manual, you can see why this works.

You could also use your amp's "FX send" as a direct out.

Thanks a lot for the very technical answer electronics has always been my weakest subject at school so I'm not very good with this stuff and and up getting get super paranoid