With version 81.0.4044.92 we were able to download attachments in custom PA forms and with 83.0.4103.61 we are not able to. Same goes for Microsoft's new Edge version 83.0.478.37. Clicking on the attachment does nothing.
I have confirmed buy installing again older version 81.0.4044.92 of Chrome and downloading an attachment works.
This is a simple PA custom form with no patching.
This is good for the communities to know so that you for making this post. However if you are looking for a workaround or a fix this might be better to create a Microsoft Support ticket on this to help delve into this. I will include a link below; Otherwise thank you for the information I will forward it off to some colleges of mine to take a look.
If you would like to create a ticket with Microsoft Customer Support here is a link on how to do so: https://docs.microsoft.com/en-us/power-platform/admin/get-help-support
Community Support Team _ Alex Rezac
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.