Thank you so much, 

but I have identified the root cause of the issue. Once we removed our custom Input_Wrapper block that uses Placeholders, the upload widget started validating correctly.

However, I still do not fully understand why the Input_Wrapper block causes this problem and how wrapping the upload widget inside a placeholder-based custom block interferes with the validation behavior.

as has been asked twice, share an oml, and people can take a look at what exactly you are doing.

Right now, we have no idea of how you implemented that validation, of what elements are on that screen, of what you mean by "at first attempt" and "on the first load"

Sorry for the late reply.

I’m not able to share the original OML, but this is a demo where I implemented the attachment validation in the same way as in the original OML. However, in the demo it works fine, and I’m not sure why the behavior is different.

I can't seem to be able open the module, and you also mentioned that it’s working in the demo, so the issue seems to be in the actual app itself. Anyway, here’s a simple assumption maybe right, maybe wrong but you can check it in your module.

The scenario is like this: when you click submit the first time, no validation happens. After you upload an attachment and click submit again, the validation message appears. The only thing I can take from this is that the condition might be swapped somewhere, so maybe double-check the logic you used. I also assumed from your screenshot that the validation message only appears after the first attachment is uploaded, which kind of supports this theory that a condition may be swapped. 

Thank you so much, 

but I have identified the root cause of the issue. Once we removed our custom Input_Wrapper block that uses Placeholders, the upload widget started validating correctly.

Sorry for the late reply.

This is a demo where I implemented the attachment validation in the same way as in the original OML. However, in the demo it works fine, and I’m not sure why the behavior is different.