Some characters in anime are not ever given surnames by the production company. This is the case for Shiina and Tsuyuko, according to Staff and Cast page of the official website:
キャスト
- 町子リョウ：佐藤利奈
- 森野きりん：大亀あすか
- 椎名 [Shiina]：小松未可子
- 内木ユキ：井口裕香
- 露子 [Tsuyuko]：小林ゆう
- 町子明：野中 藍
You can see in this list how these two names do not have surnames, whereas the rest of the characters' names in the list do.
It is not standard or commonplace in anime to start a character without a surname in the cast list and add a surname for the character at a later point. As such, we can have no reasonable expectation of this happening for these characters, and thus, it is not the case that the surname is simply "not yet revealed."
In other words, the surnames are not "missing" or "kept secret." They simply do not exist.
The only chance of the characters receiving a surname is if the mangaka adds mention of one into a currently-running manga: for example, by creating a character profile printed in the tankoubon (graphic novel volume release) or printed in a special feature page in the manga magazine it runs in, or if it comes up incidentally within the manga itself, such as meeting/greeting a character's parents, or showing the character's name on their shoe locker at school (even if this happened, it would not retroactively give a surname to the anime version of the characters). However, Koufuku Graffiti is a yonkoma (four-panel comic strip type of manga), in which it is commonplace for supporting (and even some protagonist) characters not to have any surnames. Here are some examples of such: Animal Youkocho's protagonist has a surname but none of the other characters do; Kiko-chan Smile's protagonist and her family share a surname, but other characters bear either a given name (i.e. first name) or a surname but not both; Sazae-san has three main families who have surnames but supporting characters are a mix of 1) those who have only given names, 2) those who have only surnames, and 3) those who lucked out with both.
