Dracaena Marginata….Is this cane rot? I did not find root rot when I repotted them..? HELP!

Originally, this came in one pot with two other dragon palms (small, med, tall), purchased just two months ago. The medium sized one died a terrible death when I picked them up last week and every single leaf fell off the medium palm, the leaves and cane looked similar to what is happening to this smaller one…. I have the 3rd, tallest one, in another pot and I'm assuming whatever this is will come for that one, too… I did check for root rot when I repotted them and that was not the issue.

The leaves came off of one of the branches of this smaller one last week —and then I got sick and just forgot about the issue (aaaahhh!!) and it's kind of scarred over…ouch. You can see a difference in the paler, whiter color of the cane on the recent growth of the two surviving branches, versus the measurable continuous growth they've had prior to the one branch going dark…I have no idea if that is just new, natural healthy growth or if the pale color means death?

I'd read in another post/thread that it could be cane rot? We have an average humidity of 15% where I live- I don't know if this needs to be added to my humidifier collection, but if it's 'rot' then I am stumped if it is the lack of humidity that is the problem?? …..Somebody, please, educate me…

LIGHT indoor, north facing room with big windows

WATER Filtered, 1x weekly…maybe too much watering?

SOIL palm soil, w/ 1/4 perlite added. I always aerate the soil with a chopstick after watering- and to check moisture

POT This little guy is in a nice glazed pot now, for 10 days, but, originally, I'd kept all 3 in a (new) grower's pot with good drainage.

FOOD No fertilizer, yet

HUMIDITY naturally averages 15-20% here – should I be adding this to my high humidity collection with the humidifier?

First dracaena marginatas….help