I've tried some fungicidal spray and/or neem oil in the past that seemed to have upset it more than help.

==>>> they usually do;. when you are winging it ...

im thinking cultural issues ...

when was it planted.. what is the native soil.. did you amend the planting hole ... is your soil neutral .. does this plant need something else ...

when the oldest leaves yellow.. to me.. that indicates a lack of water.. at some point since those leaves came out ... and it could have been minor enough.. that it took a few weeks to 'show' ... and then you are left guessing.. when it happened ...

the other issue on culture could be.. too much sun.. on a plant that was planted this spring ... and it just didnt have the root mass back whenever the water issue occurred ...

so you are left guessing about a combination of problems over time ...

my bottom line.. if its flowering happily ... it got over whatever happened.. and you just need to enjoy it .. and quit slathering on remedies for something that may have happened weeks or months ago ... always look to the health of the new growth .... rather than fixating on the old sacrificial leaves ...

is there mulch??? .. i dont get the whole structure its on ....

ken