The company is expanding its test of a different placement of the message icon in the main app UI, as DMs continue to see more usage in the app.

some users are now seeing the DM icon in the middle of the bottom function bar in the app. Which is currently where the "+" creation button is for regular posts, but according to IG chief Adam Mosseri, people are now messaging a lot more than they share feed updates.
"As told by Mosseri:

Messaging is used a lot more than the create button, so we're testing putting DMs in both in the middle position, and in position two where you can still swipe over to it. We'll see how it goes."

So some people are now seeing the DM icon in the middle, while others are seeing it in the second spot in the lower function bar.
This version was spotted in testing back in July, so the experiment in moving the DM icon has actually been live for some time, though it’s now seemingly being expanded to more users.

Which, as per Mosseri’s note, make sense. Over time, more and more social media interactions have switched to DMs instead, with Mosseri noting this shift back in 2022.
Which is why Instagram has been working to lean into that shift by adding new features like inbox Notes, which will encourage even more interaction in the DM, and making it easier to share posts through DMs.

While messaging isn't necessarily what IG originally did, it's now sort of what it is.

As Mosseri has said, during an announcement earlier this year:

When you think of Instagram, probably what comes to mind is a feed of square photos. How Instagram works has changed a lot in recent years; if you look at what people share, and how people express their creativity, the primary way they do so is actually DMs on Instagram. More photos and videos are shared in DMs than are to Stories or to feed on any given day.

So more than what Mosseri wrote in 2022, for example, DMs are actually now the primary way in which users are getting to engage with each other, with the main feed more of a TikTok-like stream of recommended entertainment.

As such, the change probably makes sense, even if it does seem a bit unusual. It might not actually be necessary, but perhaps it helps to fuel already rising behaviors.