Using lvgl v6.0, but as far as I can tell, this is still present in current v7 too.
In lv_page_set_sb_mode(), if called with LV_SB_MODE_UNHIDE, it may exit on the
if(ext->sb.mode == sb_mode) return;
line, if it was called previously with LV_SB_MODE_ON and then with LV_SB_MODE_HIDE.
An easy fix is to set LV_SB_MODE_UNHIDE=8.