I get my best support answers when I say I do not know

Someone posted a query regarding export timeouts on files over roughly 40MB. No clue whatsoever. I would've previously written a response along the lines of how much I appreciated his patience while I checked it out.

I just admitted that I did not know what the problem was, told him what I wanted to look at, and if it was alright to share the problematic file with me. Four minutes later I received the file and found out it was a 30 second hardcoded timeout I'd written way back in 2024, probably forgot about.

He's now reporting close to a third of the bugs I encounter. I'm sure it's not because I resolved the issue quickly but because I stopped acting competent to him.

The part I find most difficult to grasp is how honesty comes across as unprofessional. Telling an enterprise client "no idea" in a trial is definitely different from telling a hobbyist that.

How do you distinguish between the two? I'd prefer learning from someone else's mistake rather than my own.