ffmpeg – How to correcty burn/overlay teletext subtitles as a bitmap-based subtitles?

I have a TS file with a complete teletext stream. In page 888, there are subtitles.
I managed to overlay them as I wanted using ffmpeg -txt_page 888 -i test.ts -filter_complex "[0:s]scale=1920:1080[subs];[0:v][subs]overlay[v]" -map "[v]" -map 0:a output.ts

However, many subtitles are missing and some of them linger for a bit.

In VLC (that also uses zvbi), all subtitles show up correctly.

What my be causing this problem? Is there a correct way to do this.

PS.: I don’t want to extract them to .srt or using it as middleware and then burning them, as they will loose some special aligments. I want to burn zvbi’s output onto the video.