Issue when adding a single pole switch next to a 3 way

Question

Hoping for some help here.  I have a 3 way dimmer switch running to a fan/light combo that I have jumped a single pole switch off of to power a new exterior light.  There are two 3 way switches on the fan/light combo and I have source power coming into the box which ran through the 3 way to the fan/light combo which I jumped off of.  I pigtailed the hot off the source and ran one to the 3 way and one to the single pole switch and tied together the neutrals.
The issues I am having are:
When the fan/light combo is off and the exterior light is on, one of the 4 lights on the fan is on with low power. Also, when I pull the chain and shut off the fan light, the exterior light goes off.
When both the fan/light combo and exterior light are on, all is good.
Both 3 way switches are wired correctly, though I haven’t been in the fan yet (hoping to fix without opening up the fan connections since the wife wants a new one and I can’t justify taking it down and putting the old one back up).
What is the issue here?  Issue with the 3 way dimmer, issue in the fan, the “source” is something different altogether?
From a comment:

So ironically, In digging further, it seems my source into the box has the line power on the white wire and neutral on the black wire. One thing I am confused about though ... it appears the hot travels up the fan/light and neutral connects to the common on the 3 way switch ... fan/light and 3 ways work as intended with the setup but that wiring doesn’t seem right. For the new light ... as long as I tie the neutral to the black source wire and the hot side to the white source wire, I should be good right?

Harper - Reinstate Monica · Answer

REWRITE:
Having the 3-way travelers go through the fan box is quite unusual, but I was "going on faith" on that.  However, given the behavior of the supposed "supply" wires you mention in comments, notably white being hot and black being pulled down to 0V... it's clear this is a 3-way switch loop.  Neutral is not present at the box at all.
I recolored it to designate wire function (black=always-hot, red=switched-hot, yellow=travelers)... because 3-ways are too confusing otherwise.

Note the complete absence of neutral from this box.  White is an always-hot wire (as is required by Code; if neutral is not present in the cable and always-hot is; white must be used for it; that makes it easier to detect as a hot.  It's odd that the person knew this rule yet did not use it in the other switch leg).  Black in your circuit is switched-hot to the fan - i.e. it powers the fan.  The fan's load is "pulling it down toward neutral" which is why it reads 0V.  It is not neutral.
If you have any doubt of this, connect that black and white to a receptacle and plug in a 1500W heater or toaster (simple heat appliance, but high in wattage). I guarantee the fan/light will activate if you do.  Switch loop!
Neutral is not present in an old style switch loop like this, so branching an additional light off it is not going to happen. Sorry!

Issue when adding a single pole switch next to a 3 way

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