I have had similar damage,

quite sure that it is some fungus, clogging the system "pipes",

most often in the phase when I harden off Dahlias sprouting in pots, they would stand in full sun and some got this

IME

if it is just one shoot, the next one can stay healthy,

so, pull off the diseased one and bin it, wash hands...

I would try to avoid stress for them, keep up with the cautious / little watering, frequent airing,

do you start them inddors or are they in their beds?

Best of luck, Lin

I have them all in pots!

For whatever the reason, this use to happen to me all the time when I'd use a real peaty soil that I had obtained from a low spot on my own property.