Did you remove the excess padding from the flac file?
That can be achieved by for example running this command using the flac cli on your file.
flac --best --verify --force --padding=4096 *.flac
Note that this only works if you have the flac.exe in your PATH.
What the command does is reencode all .flac files in the folder in which it was executed with maximum compression, overwrite the existing files and set the padding to 4096kb. That removes excess padding left behind in the files from a large embedded image.
Alternatively, if you have mp3tag installed, you can use the new option described here instead to only adjust the padding size without reencoding.
Short version: Drag the song into mp3tag, right click it and use Utils → Optimize FLAC which will also set the padding to 4096kb.