Like many things, it's one of those "it depends" things, because gag strips or slice of life comics can get away with a lot more of this kind of thing, but of course I have my general rule of "no extras in the middle of a chapter", but even that, if I was making longer updates than an average of about 10 panels, like if I was making a long scroll comic where an "episode" is almost like a chapter, I wouldn't necessarily think it was terrible to put an omake or a piece of fan art at the end.
If I had to distil it down to a "rule to live by" I'd say this:
The reader should never click your comic and feel disappointed by what they get for doing it.
If you keep putting scrappy doodles or irrelevant gags into the middle of your story, you train your subscribers to feel like "ugh, whatever it can wait" about your updates, and if you put off moving the plot forward too long, they might even lose that sense of excitement about what happens next. If you want to build an audience, you have to keep people excited about your story and make them feel like, "oh hell yes! Update!" when they see that notification and jump right on it.
A common issue is people who think of their characters as "OCs" who want to short cut over to the readers caring as much about their characters as they do, so they start making like... Q&As, side gag sketches, holiday dress-up art and all sorts of things that would appeal to a fan of an established character that fans know and love...before they've established the character in the story or made the reader love them. You can't just take a short cut and skip to the part where we're invested in the characters; you have to put in that work of introducing them, letting the reader follow them through tricky, intense or funny scenarios and building up that relationship readers have with a character by making pages first before you can get a response just by putting up a picture of the character in an amusingly appropriate cosplay, or a 4-koma comic riffing on funny stuff that happened in the plot.
Pages are nearly always better than extras. Extras should either be something readers get in addition to pages, or the audience should be aware that what they're getting is filler to tide them over for more pages, like during a hiatus or between chapters or seasons. If it's an illness or something and you let them know there will be fillers for a while, readers will understand, but if it becomes normal to click an update, expecting story content and then, "Oh... it's just another sketch of the characters", don't be surprised if people stop clicking. Always be thankful for people clicking your comic. Never take it for granted.