Animation > Mp4 > Facebook : bad quality

Hello all,

I designed a small animation on adobe animate that I would like to broadcast in advertising mode on facebook and instagram.

It is 100% vector. The dimensions of the original file are 2160x2160px. Or 1080x1080px. I have tested both and the result does not change.

In both cases :
. in .fla the quality is nickel. The export in mp4 video via Media Encoder is also nickel.
. but when i import it into ad manager on facebook, the quality is about good on iphones and ipad and degraded - and even bad - on computer screens ... 😞

However, the video has all the qualities required for Facebook:
Settings in 2160x2160: H.264 compression, square pixels, 2160x2160, IPS: 30, 9.6MB, bit rate: 4.60Mbit / s
Settings at 1080x1080: H.264 compression, square pixels, 1080x1080, IPS: 30, 4.6MB, bit rate: 2.9Mbit / s

Is there something to do in particular so that an animation on adobe animate transformed into mp4 video is displayed nickel on Facebook in desktop mode?

Thank you so much !