Factorio Forums

Types/Sprite shows...

mipmap_count
Type: Types/uint8
Default: 0
Only loaded if this is an icon (has flag "icon").

but I've seen in it used in base game with flags other than "icon".

Looking at Types/SpriteFlags there is a "mipmap" flag... is this relevant?

No, the mipmap flag is not relevant. The relevant flags are "group=gui" and "group=icon". Some other flags include these, that is documented on https://wiki.factorio.com/Types/SpriteFlags.
What isn't documented anywhere is that sprites can have default flags set by the game. For example https://wiki.factorio.com/Types/BorderImageSet has the "gui" flag set on its sprites by default (which sets "group=gui"). This means that these sprites can use mipmap_count even though you don't *see* the flag being set anywhere.

TL;DR: The flags are "group=gui" and "group=icon", I updated the wiki to reflect that. The behaviour you observe is related to default sprite flags that aren't documented anywhere.