I have the same problem!
If I click on where the caption should be in Gutenberg editor and drag the cursor, it highlights the caption text. The caption text is white and I have a white background for my theme – so I think the caption text is there – it is just white on white!
On the actual page there is no problem and the caption text is black (on white page) – it is just the Gutenberg editor page that has the problem.
This only started to happen recently.
I updated to WP 5.7.2 recently.
I have a second WP site which is also 5.7.2 but this seems to be OK.
P.S. Forgot to add – if I highlight the text and change its color to black (it was already set to black) then Gutenberg will then display it.
