Text Highlight keeps resetting when I switch to Compose mode

Hi, I’ve got myself into a bit of a mess with text highlighting and I was wondering if anyone could help me reset it please?

Every time I switch out of Compose mode the highlight colour changes to black and I have to toggle it off and change it to a different colour.

I think the problem first occurred after I control-clicked on the text highlight option in the format bar an chose ‘show colors’ then chose a red background. Now, no matter what I do, the highlight colour resets itself to black when I leave the Compose mode.

If I reset the highlight colour and toggle it off, then when I enter compose mode the highlight is changed to black. If I set the colour to any other colour, leave highlighting on and enter compose mode, the correct colour is retained.

It seems that I have somehow set the default to black but can’t see a way to change it. The default is set to yellow in the preferences.

I wondered if it was perhaps a bug due to the problems described in this post:

https://forum.literatureandlatte.com/t/sections-mysteriously-highlighted-in-white/35390/1

Thanks in advance for any help.