Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi, I'm Diane and I'm happy to help you today.
This is happening in classic Outlook? What is your line width? Here, I can repro it on some images - on those, it is either too narrow to be visible or not as wide as the other sides, until I increase the line width. 1.75 pt seems to be the magic number for most images I tested - the bottom line is visible but it might be narrower than the other sides - in that case, 3.25 pt seems to be the magic number to have the line the same width.
But it doesn't happen with all images - some are fine with 1 or 2 pt line.
Let me know how it goes, I'm here to help you further if needed.
-- Diane