Micro.blog strips EXIF data, which can include an orientation baked in because images are weird. You’ll want to use mogrify from ImageMagick to fix this (or if you’re using convert already in ImageMagick, you can add --auto-orient to fix.
This is a similar complaint to mine about color spaces: Colorspaces still get stripped pretty brutally - #5 by yurymol
I think Micro.blog is way overly aggressive on EXIF data being identifiable, and instead specifically remove only categories like the lat and longitude and not those like orientation, color space, etc that influence the display of the image.