Follow the property through the pipeline
A PNG with transparent space establishes a property of that image. It does not prove that the animation frames, video encoder, container and destination import preserve the same property. Record the actual path and inspect each consequential stage.
Use a tiny owned sample with opaque, fully transparent and partially transparent regions. A checkerboard in an editor may be a display aid, so confirm what the exported asset contains rather than taking the preview background as evidence.
MDN's VideoFrame format reference distinguishes alpha-bearing representations from opaque padding formats. That explains why four components alone do not necessarily mean useful transparency. Raw-frame representation still does not certify a particular compressed delivery format.
Find the earliest changed stage
Check the source artwork, prepared frames, encoded output and destination composite in order. If alpha survives encoding but the recipient shows black behind it, investigate import and display behaviour. If it was removed before encoding, changing the recipient's settings cannot restore it.
Keep each candidate labelled and write to a new output. Renaming an opaque file or remuxing unchanged streams should not be described as recreating lost transparency. Return to a source stage that still contains the required information.
VideoFrame: format property by Mozilla Contributors is available under CC-BY-SA 2.5 or later. The staged review here is original guidance. If the destination cannot accept transparency, deliver a clearly labelled composited fallback and preserve the alpha-bearing master for another workflow.
Questions answered
Does a four-component format always contain usable alpha?
No. Some representations use an opaque padding component.
Can changing the extension restore discarded transparency?
No. Return to an asset that retains the information and choose a supported output path.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.