This issue was originally caused by work done in Bug 1709129. The implementation there had it so we detected potential "spam" from a website, so it would allow the first download to happen but any subsequent ones would be blocked if they weren't evaluated to be user-initiated. There's still work we need to do for this so it was moved behind a separate pref in Bug 1731668.
Bug 1734903 Comment 4 Edit History
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This issue was originally caused by work done in Bug 1709129. The implementation there had it so we detected potential "spam" from a website, so it would allow the first download to happen but any subsequent ones would be blocked if they weren't evaluated to be user-initiated. There's still work we need to do for this so it was moved behind a separate pref in Bug 1731668, which was landed in 94.