Other than syphilis, are there any sti or std known to cause scleritis?

No, none. And as you were told yesterday, syphilis rarely causes it. Of every thousand cases of scleritis, I would guess that no more than one or two would have syphilis as the cause. And your own syphilis tests prove it isn't the cause in your case. Keep working with your doctor(s) about other explanations.

Scleritis is most commonly seen in autoimmune disorders as well as other rare conditions such as sarcoidosis. Among infections herpes zoster seems to be many times the culprit.