One theory - is the humidity too low?

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've had this happen time to time. It just seems that that leaf didn't develop properly. Eventually it'll either grow to normal size or turn brown and die. The latter is more common. Don't be to alarmed your plant has healthy new leaves so that's good.

I did remove a stake ( can’t remember if it was in that spot) last year based on advice from this board. ===>>>> thx for the followup .. its nice to know.. that even a blind squirrel can find a nut once in a while .... lol.. i think we are talking at cross purposes .... there are 2 different pruning options ... down lower as i was suggesting ... taking away the whole wound .... verticla and horizontal ... or just taking the smaller side branch ... and the more i think about it ... i would take the smaller branch.. so the wound on the ...lets call it the trunk .... can continue to heal .... as with all pruning ..... you can always go back and take more ... but its really hard to staple or glue stuff back on ... lol ... it would be rare that the festering wound would kill the whole tree ... so dont worry about that ... but ... properly pruning out damage.. is the usual suggestion.. not really all that big a deal .... yes.. the tree might take on a new form.. a new future ... but in the long run.. it will heal itself.. and have a long life ... and you just have to accept the new form ... but its not a harm .. anyway .. good luck.. ken