I had weird issues like that with my previous phone which was a Galaxy. It started when the phone was getting old. (I used it for 4 years). I never could figure it out. I tried different sizes of fonts and that didn't work.
I might suggest using the "desktop site" option on your phone and see if that makes a difference. It's usually in the new tab, incognito tab, bookmarks drop-down menu.
Sometimes its the poster. If you don't space your dialog, the attachment will sit over the writing. At least that is what has happened to me before both posting and trying to read other posts.
If you are going to post an attachment be sure to hit return several times before you begin typing again. Preview Post is your friend. Use it.