Documented also in
https://lists.texmacs.org/wws/arc/texmacs-users/2021-07/msg00036.html
https://lists.texmacs.org/wws/arc/texmacs-users/2021-07/msg00037.html

In (at least) some figures originated from an imported LaTeX file, there is a newline before the image tag. For example:

<\big-figure>
  <\with|par-mode|center>
    \ <image|water_layer_reflectivity|.8par|||>
  </with>

  \;
</big-figure|Reflection off a water layer on top of
silver<label|fig:water_layer_silver_reflectivity>>

Under these conditions, preview-ref shows the "question mark" icon instead of showing the figure caption.