Our 6 week old daughter (born at 37 weeks IUGR) has started to rotate her body in her bassinet- not roll onto side. We will put her in safely- vertically- then wake a few hours later to see she has partly rotated and turned her body so she is more horizontal with her head moving towards the side of the cot almost on a 90 degree turn rotation. She has been using a Love to Dream zip sleep suit* so she can sleep with her hands up but be secure- she didn't like the traditional swaddle wrap. She has a tight blanket tuck on her lower half on all 3 sides of the bassinet to secure her as well. Shes just reached 3kg and is going through a bit of a 'leap'.

Some babies do not respond well to swaddling & some like their arms free to touch their face or suck on fingers.
Many of these babies will wriggle & squirm in an attempt to get arms free, & often their swaddle comes loose, or they end up in various positions in the bassinet.
With swaddle suits, like the one you describe, they can’t “escape” & often end up in different positions.

There are many different types of swaddle suits - but they are all classified as swaddles, ie the baby’s arms are tucked in & not free.
You may like to try using a muslin or cotton swaddle but wrapping with arms out as in ink below.
https://rednose.org.au/article/wrapping-babies
In this same link is also some information “wrapping & swaddle products” that you may also find helpful.(scroll down page)

You could also try her in a sleeping bag (arms out)
If she continues to wriggle squirm, you could also try her in her cot.

It is preferable to use the blanket for warmth/comfort not as a restraint - as you see she is still moving around with the blanket on. This can potentially create a risk of head/face being covered.
If there is any risk of the blanket covering her head/face then it may be necessary to stop using.

Have you started tummy time yet?
How does she go? - if you haven’t been doing tummy time yet, it is important to start.
https://rednose.org.au/downloads/RN0407_22_Tummy-Time.pdf

Always continue to place baby on back for sleep.

She may also be getting close to trying to roll.
Most babies are rolling by 4-5 mths - some earlier, some later.