Is the file extension of the thumbnail in uppercase. ie PNG. If so this could be the cause, as we have a known limitation for this (discovered a few weeks back) and therefore the thumbnail name and extension must be all lowercase. For 3.4 documentation update, we have added the following Note to this section:
Survey123 Connect requires all image files used for thumbnails to have a lowercase file extension. Uppercase file extensions are not recognized by Survey123 Connect, and these thumbnails won't be published with the survey.
If this doesn't resolve your issue, can you please send an example .png file that is not working so we can take a closer look.