Spell checker affected by formatting

[borekb]

I have this Markdown:

It's still _a lot_ of work.
It's still a lot of work.

The first line reports "of" as a spelling error, the second line doesn't:

[borekb]

I don't always see this problem, for example, here's a similar phrase in a different context:

[astoilkov]

Thanks. I will look into this.