Mosaics on videos is normally poor bitrate.
This can be due to a multitude of reasons but normally it's the transmission side of the stream that is the cause.
For example, when mobile phones have poor bitrate, the video will pause until it buffers more, but if the tramsmission has poor bitrate, then the devices will play the transmitted video in any form, in the case you see these blocks, it's because the full frame data hasn't been sent in time for the device to decode.
This is usually down to a busy server and can be expected to be temporary.
