Per my test, I have the same result with you when we select the multi-line column type as Rich text.
And per my research I found this is by design. We can't edit the multi-line text with rich text (Bold, italics, text alignment, hyperlinks) type in datasheet view.
As a workaround, change the type to Enhanced rich text (Rich text with pictures, tables, and hyperlinks) instead of Plain text.
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