It appears Instagram is experimenting with a new ad format for Reels ads, and there is a new variation of Multi-Advertiser display which now is showing to some users
The new multi-ads format groups a selection of related brands onto a single screen and is then displayed between Reels posts.

The concept itself is nothing new. Instagram first rolled out Multi-Advertiser Ads for its main feed back in October, with a side-scrolling display of related ads shown based on user engagement.
below that first post, multi-advertiser promotions provide you with multiple, related ads based on your interests.

As of Instagram:

Multi-advertiser ads help the user to discover and compare products from different businesses. This ad unit is available for select placements on Facebook and Instagram, giving advertisers a chance to be discovered by users who have recently expressed interest in related products or businesses.

Multi-advertiser ads can also be served up in the Facebook feed, and in Facebook Stories, so it's little surprise to see Meta also look to expand it to Reels as well, though it is worth noting for advertisers who opt-in to this placement.

Which is optional:
So, you do not need to turn your ads on so they appear in this way, but Instagram is interested in offering multi-advertiser ads on Reels and, if you do turn it on by ticking the box, this is what you will be paying for.

That may be advantageous, providing lower cost, higher volume placement, potentially, among people who have already seen related content. But it may also mean less exposure, and therefore lower performance, since you'll be surrounded by more brands in one frame.

Each brand will receive varying response, but again, if you are opting in to multi-ads placement, it appears this is how they'll look within the Reels feed.

We asked Meta for more on the current test of this display and whether it has any more information related to an eventual rollout. Update- This post when we hear back.