Installed packages not available on dynos

[Ruffle]

I managed to get the package I needed installed during deployment:
(specified link to .deb in Aptfile)

However the application still behaves as the package was not present
ERROR 2023-03-24 11:55:18,575 [images] Failed to load image at "<redacted>.webp" (Pixbuf error: Unrecognized image file format)

When I use bash and check if the package is present:

~ $ apt list --installed | grep pixbuf

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

libgdk-pixbuf2.0-0/now 2.40.0+dfsg-3ubuntu0.4 amd64 [installed,local]
libgdk-pixbuf2.0-common/now 2.40.0+dfsg-3ubuntu0.4 all [installed,local]
~ $ 

Is this normal? Isn't the whole point of this buildpack to have the package available on runtime and not only during deployment?